The 6MH6 has an equivalent base pinout, so it should be usable, although
they are not common.

You can build an adapter to change the pinout to a more conventional tube
for testing. Tube testers can be tricked quite easily.

If the tube is being used in a transmitter, it is not that difficult to
change to a different type of final tube - including the 6146. If it is
used in a grounded grid amp, the grids are all tied together, so you should
be able to find a substitute and change a few pins on the amp.

Hams are supposed to be good at improvising.

73, Colin K7FM