The 6CM6 is a low frequency power amplifier beam tetrode. It can be used as a Class A audio amplifier but was mainly designed as a vertical deflection amplifier in 525 line TV receivers. The 6BW6 has similar characteristics but different base connections. The characteristic curves match the 6V6 family and the 6BW6 is essentially the 6V6 on a B9A base..
The the B9A base and lower mica.
The anode obscures all of the internal structure. This was made in the USA.
Thanks to Mark Mallison for sending these images.
The thin glass tube envelope is mm in diameter, and excluding the B9A base pins is 63 mm tall.
Reference: Data-sheet. Type 6CM6 was first introduced in 1954.