From 1930 onwards, and especially from 1932, after F. Barker & Son was taken over and became F. Barker & Son (1932) Ltd., the letter "B" was applied as a prefix to the serial numbers of all military compasses manufactured by F. Barker & Son.
By that time Barker had gained almost total monopoly of compass sales to the British War Department (nowadays Ministry of Defence) and also jealously safeguarded its patent rights. Eventually Barker licensed the rights to produce the MkIII compass to a company in Canada and also one in Australia, but all MkIII compasses manufactured in Britain up until they ceased production around 1966 (then becoming the Mk1 until 1971), were manufactured by F. Barker & Son.
Even Stanley sold "B" prefixed MkIII compasses for a while in the 1950's and 1960's and also a few Mk1 compasses from 1966 onwards, before finally developing their own G-150 model (and various other prototype models such as the very rare L1A1) and muscling into the British Ministry of Defence market. For a while Stanley even managed to gain lead position in sales of compasses to the British Ministry of Defence with their own G-150 model, but it was fairly short-lived and Barker regained ground. Eventually the Stanley (SIRS) G-150 ceased production and Barker, who had held a huge share of the worldwide market for many years, retook the UK market with the M-73 and the M-88 models.
Various other makes of MkIII can be found dating from WW2 until around 1971, such as H.B.& S. (Henry Browne & Son), GEC, TGCo and even Stanley, but all of them have the "B" prefix to the serial number, except for those produced under license in Canada and Australia. It would seem that the British Armed Forces were reluctant to purchase any compasses that did not carry the "B" prefix. Barker had almost, if not total monopoly of compass sales in those days, and is still the principal supplier of compasses to Her Majesty's Forces to this day.
So, TGCo. (The Gramophone Company - part of the EMI Group), GEC (General Electric Company), HB&S (Henry Browne & Son), J. M. Glauser & Sons, Stanley and many other names you see on the back of MkII and MkIII (and the later Mk1 and also the 1939 and 1940 models of the Verner's Pattern MkIX) compasses did not actually produce the compasses themselves, but acquired them wholesale from Barker. Barker had patent rights to the MkIII design and had such an efficient mass production facility that it was able to produce vast numbers at relatively low cost. It would not have been cost effective for another maker to tool up and manufacture MkIII compasses, as such an operation would be much more costly and labour intensive than simply buying the compasses direct from the principal manufacturer. Effectively sensible business practice that is still used by companies to this day. Likewise, Barker sold telescopes, binoculars, thermometers, drawing instruments and many other items it never actually produced itself, but those items were all branded with the Barker trademark prior to sale, and the customers believed they were buying Barker products. This is effective and good marketing.... make the customer believe. After Barker stopped producing the MkIII (Mk1) around 1971 J. M. Glauser & Sons did actually develop the Glauser MkIV, but few were made and they never gained British Ministry of Defence or NATO accreditation. Stanley went on to produce the L1A1, the G-150 and a series of other prismatic compasses, and other players, such as Silva with their Mk4, entered the prismatic compass market.
Many other earlier compasses signed by famous names, such as Negretti & Zambra, John Lizars, C.W. Dixey to name but a few, were actually manufactured by Barker. F. Barker & Son was never a retailer, and this is still the case today. Up until the 1980's Barker's two principal markets had always been the British Ministry of Defence and foreign armed forces (military) and also wholesale to compass retailers. Nowadays Barker no longer produces for wholesale, but instead concentrates its efforts on the supply of compasses to British and foreign armed forces.
