The Red "Passing Light"

My BT-13 has what's called a "passing light" mounted beside one of the landing lights (if I remember correctly it's in the left wing). It looks like a red lensed landing light, maybe 5 or 6" in diameter. What I don't understand however is how it was used. An old wartime C-47 pilot told me that he thought they used it at night to signal to an airfield that was strictly light signal controlled that the aircraft was a military aircraft and thus had priority over other, civilian traffic, but he couldn't be certain anymore. Anybody know for sure?

thanks,
Dan

[Randy Haskin]

The T-37 has the same thing up in the nose. It is only on when the gear is down, so I always thought it had to do with being able to confirm that the gear is down.

I don't think that's the case with the BT, though!!