FT-243 xtals should work ok. The BC-610 was designed just before WW2. At
that time xtals usually came in a round holder about the size of a
half-dollar, with two pins that fit into a standard 5-pin tube socket (the
pin spacing was not the same as that of the BC-610 xtals, though). I think
they made the rectangular style xtal holder for easier storage as would be
necessary with a military unit, but internally the construction was about
the same; the xtal was sandwiched between two metal plates, relying on a
spring to maintain a certain amount of pressure on the crystal plate. In
fact, I recall in years past I have removed the crystal plate itself from a
BC-610 holder and put it into one of the older style round holders and it
worked just fine.
The FT-243 didn't appear until during WW2. I suspect they kept the larger
style holder because that was what was already in use in the field by the
military, and they wouldn't have wanted to change styles of crystal in the
middle of the war.
I am curious if anyone is using a pre-WW2 version of the HT-4. What kind of
crystal holder did it use? Was it identical to the one in the wartime
BC-610?
Actually, it shouldn't be too difficult to fabricate an adaptor. The FT-243
xtal will fit into an octal tube socket, for example into pins 1 &3, 2&4,
3&6, etc (in other words you have to skip a pin). Just find a brass rod the
proper diameter, or maybe even a couple of pins from an old 6- or 7- pin
tube base might be the right size to fit into the BC-610 xtal socket.
Solder a wire to each prong, insert them into the xtal socket, and solder
the leads to the octal tube socket. You could just leave it dangling and
get the beast to work, but with a few junkbox parts and some basic shop
tools, it should be easy enough to turn out a permanent professional looking
adaptor.
I have seen commercially made adaptors occasionally at hamfests.
