Hello Ivan
I had a look at the P38 air file, the plane is fitted with left main tank, left right, and center1, I'm not too much clever with air files, but I didn't found any other tank, so may be the drop one is only there to be pretty? the FS2K SDK don't contains any external tank, there are eight values for fuel tank selector, and FUEL_TANK_SELECTOR_EXTERNAL1 & 2 came with FS2K2. CFS and CFS2, that came after FS2K perhaps could take in count those tanks?
All that follow is hypothetic, and I didn't check anything, and generally 90% of experimented hypotheses don't work.
So may be one can try to replace with the FS2K2 fuel tanks enum values the ones in the Gauges header, and experiment, in the worst case it won't work and might not crash the sim.
In retrospect, the P38 fuel selector is a needle, with a linearity table with six values, so you could test in the draw part:
case FUEL_TANK_SELECTOR_EXTERNAL1, return x; //x is the number you assigned in the non_linearity_table to this point
and in the mouse part:
trigger_key_event( KEY_FUEL_SELECTOR_SET, FUEL_TANK_SELECTOR_EXTERNAL1 );
if the needle don't want to go there, it is the sim don't take care of this tank.
I dream, I dream, but sometimes such silly idea works and at this time, you are very happy with what you discovered
Cheers
Martin