Time Spec: The Mark XI

Perhaps more than any other watch in history, the Mk. 11 truly embodies what it means to be a “tool watch.” Manufactured by International Watch Company and Jaeger-LeCoultre based on specifications provided by the British Ministry of Defense (MoD), the Mk. 11 was designed from the ground up with functionality and utility at the forefront. The result? A quintessential pilot’s watch, and one that would inspire countless designs years after its humble beginning in 1949. Today, we take a closer look at this historic piece.


Image via Franco Squelette.

In the early 1940s, the MoD created a set of specifications for watches to be issued to military personnel. 12 companies were awarded contracts: Buren, Cyma, Eterna, Grana, JLC, Lemania, IWC, Omega, Record, Timor, Vertex, and Longines. The resulting watches–highly valued collector’s pieces in their own right–would eventually be called the “Dirty Dozen,” and to a lesser extent the Mk. X watches.

“Dirty Dozen”; Photo credit: user siewming, Malaysia Watch Forum

Though the watches issued under the Dirty Dozen were approved for military use, their lower level accuracy proved to be too unreliable for aviation. So, sometime in 1946 or ’47, the MoD decided on a new navigator’s watch standard—6B/346, also known as the Mk. XI/Mk. 11.


Mark XI specificationsThe requirements specified the following:

1) The matte black iron dial was to be “marked with Arabic numerals from 1 to 12” and was to have minute markings “in white, with the exception of the four cardinal graduations, which are luminized”; 2) a high-grade 12-ligne Swiss movement capable of a 36-hour power reserve and a daily rate variation of no more than -4/+4 seconds. It should also be equipped with a Glucydur balance, a Nivarox hairspring, a centrally mounted seconds hand, and a hacking function for synchronization; 3) a case waterproof to 20 feet; 4) magnetic shielding via a Faraday cage; 5) an acrylic crystal held together via a retaining screw to prevent it from detaching during decompression; and 6) a stainless steel “Bonklip” bracelet (which had its own military reference number 6B/2763) permanently fitted to the case.

The MoD took this list to London-based jeweler Goldsmiths & Silversmiths Co., Ltd. They in turn reached out to various manufacturers, and IWC and JLC eventually secured the commission. Issued in 1949, the Mk. 11 was supplied to the RAF, FAA (Fleet Air Arm), RAAF (Royal Australian Air Force), RNZAF (Royal New Zealand Air Force) and a handful of other outfits, and eventually even some private firms.

Before being issued for service, all watches had to endure rigorous performance testing at the chronometer workshop of the Royal Greenwich Observatory in Herstmonceux, where they were regulated to five positions and various temperatures over a two-week period. To maintain optimal operation, watches had to be retested annually, regardless of whether or not they were ever issued.

L: IWC; R: JLC. Image via Franco Squelette.

Given the stringent specifications laid out by the MoD, both Marks from IWC and JLC were quite similar in build and appearance. Both came in stainless steel cases approximately 35-36mm wide–35.3mm for JLC, 35.98mm for IWC. The cases had long lugs and most featured fixed spring bars. The dials were cup-shaped and designed to lock in with an iron back to create an anti-magnetic seal, or a Faraday cage. Both JLC and IWC manufactured a handful of Mk. 11 variants during the years they were commissioned, boasting several different (although not too dissimilar) dials, handsets, and case back markings.

Over time, the Mk. 11 underwent some notable design changes. The first generation of watches, for example, did not feature a triangle at 12, but rather an Arabic 12 flanked by two dots. This was later changed around 1952 “to improve clarity of presentation.” Likewise regarding the handset, with the shorter, squared hour hand (now iconic to IWC and the Mark line) being preferred for immediate legibility. Starting around 1962, IWC Mk. 11s featured an encircled T on the dial above 6 o’clock to indicate the presence of tritium (the MoD began to issue tritium replacement dials for older radium versions around this time). The circle T did not appear on RAAF and RNZAF versions of the watch because they did not request it. The aforementioned bonklip bracelet was eventually discontinued and replaced with a nylon NATO strap (6B/2617), only to be reintroduced years later as an alternative.

L: JLC; R: IWC. Image via Franco Squelette.

Each Mk. 11 had to be branded with a broad arrow on the dial, case, and movement to denote government property. Likewise, each watch had to display the store reference number, 6B/346. 6B signifies “Aircraft Navigation Equipment, Accessories and Unit Servicing Parts,” a designation not exclusive to watches. 346 was the chronological number. Each unit also had to indicate a serial number and the year the batch was ordered. For example, 2283/51 indicates the 2,283th unit from a 1951 order.

L: IWC Cal. 89 (Photo credit via Watchxchange); R: JLC Cal. 488/SBr

The JLC was equipped with the JLC Cal. 488/SBr, a chronometer-grade movement with few contemporary peers. The caliber was produced in very low production runs for a short period of time, and its design would eventually become the basis for the cal. P478/BWSbr used in the iconic 1958 JLC Chronomètre Geophysique. Quite the lineage.

The IWC was fitted with the renowned Caliber 89, widely regarded as one of the most robust three-handed movements ever built. The Cal. 89 runs at 18,000 bph and features a Breguet hairspring and a patented indirect drive for the centrally mounted seconds hand. The first generation of Cal. 89s lacked shock-protection, a deficiency that would eventually make the JLC Mk. 11 obsolete. After realizing the cost to repair damaged watches was greater than the cost to include shock-protection from the start, all subsequent productions included Incabloc.

As mentioned, JLC’s Mk. 11 was discontinued in 1953 largely due to its inadequate shock-resistance. It is believed that around 2,950 units were made for the RAF (and the later the RAAF), and all were decommissioned in the early ‘60s. IWC became the sole supplier of the Mk. 11, and the firm produced nearly 8,000 units between 1949 and 1953. The Mk. 11 was finally decommissioned in 1981, and approximately 1,000 units total were also sold commercially in 1973 and 1984. There were no commercially available JLC units.

JLC Mk. 11 variants. Photo credit: Analog Shift

For the sake of brevity, I won’t get into all of the variations that exist within the Mk. 11 line. The information is simply too dense for any general overview, and some of the details can get quite a bit murky. Most of the discrepancies are a result of different production runs and tweaks made along the way, inconsistent specifications from different agencies (e.g. unlike the RAF, the RAAF did not require dials to have a Broadarrow or an encircled T), and things get even hazier when you account for IWC restorations with original cases and movements and NOS replacement dials and hands. I will, however, say that one of my favorite configurations of the Mk. 11 is the RAF-issued IWC with the blunt square hour hand and a later replacement circle T dial. It’s such an iconic look, and it’s one that would inspire countless designs from IWC, among them the JLC-based Mk. XII and the ETA-powered XV–two civilian models.

Photo credit: Analog Shift

Today, the Mk. 11 is a highly sought after piece of horological militaria. Ten years ago, you could acquire a fine example for anywhere between two and three thousand. Today, you’d be hard pressed to find a quality specimen for less than eight grand. Nevertheless, it’s a wonderful timepiece with a rich history, one that any serious watch lover should know regardless of ownership ambitions.

For those feeling the Mk. 11 pull, MK II recently launched the Hawkinge, a value-driven homage to this venerable classic built to MK II’s stringent standards. Stay tuned for our review.

For a treasure trove of Mk. 11 knowledge, visit MarkEleven.

Featured image via Analog Shift.

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Ilya is Worn & Wound's Managing Editor and Video Producer. He believes that when it comes to watches, quality, simplicity and functionality are king. This may very well explain his love for German and military-inspired watches. In addition to watches, Ilya brings an encyclopedic knowledge of leather, denim and all things related to menswear.