This subject was on the Board recently but be blowed if I can find it now! However as I have gone to the trouble of typing up the following, am posting it anyway. At least I THINK it was on this web site!
David D

Formation Lights in British Service Aircraft, World War Two period.

It would seem that Formation Keeping Lights were commonly installed on RAF aircraft of this period, so the early Hurricanes were not that unusual. In AP.1564B it is noted in Volume I of this publication (in introduction, paragraph 10) that “The electrical installation, by means of an engine-driven generator, provides power for navigation, identification, landing, formation-keeping (if fitted) and cockpit lamps, fuel gauge, engine starting, heated pressure head , radio supply, reflector gun sight, heated clothing and camera gun.”
Modification 273 (in Vol I, Section 4, Chapter 1, Loading and c.g. data) under paragraph 14 covers the deletion of the Formation Keeping Lamps, this reducing the tare weight of the aircraft by some 2.75 pounds.
Section 6 (Electrical & radio wiring and servicing) includes descriptions of where to find and how to remove, test, etc, various parts of the electrical system, including the Formation Flying lamp installation, under the “Early systems” heading (Pre Mod 273) within paragraph 12, although here they are referred to as “Station Keeping Lamps” here. They are described as being located in port and starboard fairing panels, and controlled by Switchbox (Type B) located in port decking shelf. The location of the lamps is shown in Fig. 1 of this chapter (again identified as “Formation Keeping-Lamps”), and the circuit diagram is shown in Fig.11 (as “Station-keeping lamps!) Interestingly the large diagrams of the Hurricane II used as a frontispiece and labelled as Hurricane IIA & IIB, and IIC do not include the lamps. However no clue is given as to how these lamps were to be used by the pilots, although the fact that such lamps were fitted to other early wartime RAF aircraft would indicate that they must have had some specific purpose, no matter what type of aircraft. I originally though that the “Satellite” aircraft for Turbinlite Havocs seemed a probable (or possible) explanation, but on finding that other aircraft also had such lamps tends to nullify this thought. Apparently Blenheim IVs and Vs also had Formation-keeping lamps (which could also be used as identification lamps by morsing or steady illumination. Unlike the Hurricane, the contemporary Spitfire versions (Mk.II, V) seem not to have had formation-keeping lamps. The Westland Whirlwind twin-engined fighter also had Station-keeping lamps on later aircraft only (those from P6984 onwards). I see that Beaufighter Mk.IIS (Merlin engines) also were equipped with Formation keeping lamps, which could be used as identification lamps, although it is probable that early Hercules-powered aircraft also had this facility.