You might check inside the instrument. A Bristol wrench was often
included in military gear under a clip someplace.

I don't have a Bristol wrench for comparison any more, but I recall
their cross sectional shape as resembling modern Torx drivers. It would
probably take an unusually long one, and if it didn't fit well there'd
be a danger of chewing the screw so it'd never come out. Might be worth
investigating, though.

Roy Lewallen, W7EL

Bill Janssen wrote:

I vote for the Bristol answer. I think I have a couple here but it would
probably take a "crime scene" investigation to find them. I have a couple of
BC221
and the newer version with a CRT.

Bill K7NOM