Hi Tim,
The reference AP on a British lamp stands for Admiralty Pattern, and indicates that the lamp is of a type developed for that division of the military. You will frequently also find lamps whose codes begin AM or 5L (for the Air Ministry / Air Force), EL (Home Office), WO (War Office / Foreign Office), and X (Post Office Telecommunications, now British Telecom).
Today the codes for Government Service lamps in all NATO countries have been standardised, to reduce stocking requirements and enable compatibility between the military organisations in these countries. These new references always take the form 6240-99-995-XXXX for lamps 50V and below, the 995 being replaced with 996 for types above 50V. On miniature lamps where there is not room to stamp the entire code, the prefix 6240-99 may be omitted. The new reference for your lamp is 6240-99-995-2228, it is still listed as a current type for active service although I would be surprised if it had been made for a couple of decades.
Anyway back to your specific request! AP666BM is a 24-Volt 4-Watt indicator lamp, most frequently employed in gunsights. B15d base, T-11 glass, 30.5mm length and 1000 hours design life. The carbon filament type is a later development which had better shock resistance properties than the original tungsten version. They were made by GEC and Ediswan (in that brand name) right up to the late 1960s. I would guess your tugsten version is quite old though, and the use of the Admiralty stock number instead of the more modern NATO Inter-Service number suggests a production date before 1955.
There is also a similar AP666M which has a bulb made in clear glass. The blue version was used for night sighting. GEC made a number of red glass versions for other indicator applications. You can find a complete catalogue of current Government Service lamps in Defence Standard 62-6, available free of charge from the Ministry of Defence in UK. I presume that the US authorities must produce a similar document but I never had any success in tracking down who is responsible for its publication. The British version certainly makes interesting reading though, and is an invaluable reference for finding specs on lamps bearing these codes.
Best regards,
James
