Because early Spitfire 3-position light switches are getting scarce and very expensive when found, I have investigated a method to re-configure one from a later Spit (up to 1976) or GT6 that is configured as a 2-position switch from the factory. The switch I used for the conversion is part number 39866A. I have provided illustrations of both the original and final configurations.
Basically, Lucas used the same casing and circuit layout for both of the switches, so the modification requires no mechanical alteration except for removal of a stop screw that locks out the off position in the later switches. The following steps will complete the modification. In order to access the circuits and lockout screw, the spring and rolled pin that hold the two parts together must be removed.
1. Remove the lockout screw that prevents the switch from actuating the third position.
2. Cut the shorting strap that connects the flash contact to contact "E".
3. Short the Flash contact to contact "A".
4. Short contact "D" to contact "E".
Note: Use a low wattage soldering iron and use extreme care in soldering the new contacts because the substrate on which the contacts are mounted will melt easily.
Switch Position Truth Table Position 1 (UP) B-C, B-F (F is not present on earlier switches)
Position 2 (MIDDLE) B-E
Position 3 (DOWN) B-A (by step 3 at left)
B-D (by step 4 at left)