Q. Instead of a 0C3 tube the unit has a 0D3.

A. Just a different regulated voltage. No magic about voltage. In fact,
the builder may have found that the different voltage may have offered
better stability - or it could have been he had one on hand.

Q. Instead of a 1626 tube the unit has a VT135/12J5GT tube.

A. Should be an equivalent tube. No big deal. A triode is a triode.

Q. The second 1629 tube has not been removed (no wire to top cap)

A. You apparently mean the 1625 power output tube. Should not make much
difference. Maybe the builder did not have a place to put it. The original
BC-457 had tubes in filaments in series - parallel so it could operate on
the aircraft 24 - 28 volts. Most hams converted the wiring to 12 volts by
paralleling everything. I recall most conversions supplied a separate
filament transformer, and as long as the voltage required is equal to the
transfromer voltage, you are fine.

Hint, if it worked once, and no one rewired it, it will still work - unless
one of the components has died due to aging.

All the BC-457 conversion does is to use the triode oscillator circuit as an
external vfo. Output from the vfo is taken from the grid circuit in the
BC-457. It is a very easy circuit to understand.

For 80 and 20 meters, you use 5 to 5.5 MHz. For some of the other bands,
you must multiply the frequency of the vfo. That was often done in the 1625
section. But, much of the operation of the CE was done on 80 and 20 and the
fellow who installed it may not have gotten any further.

Colin K7FM