Umm 6 bits is perfect for BCD, look at IBM's
1620 : 4 bits BCD, 1 bit
sign flag/length flag 1 bit parity
Very inefficient. I hope you are not serious.
I don;t know much about IBM stuff, but I do know that the HP9100
calculator had 6-bit memory locations (this is obvious from the
schematics of the memory data board, from the fact that the main CPU
register (register '2', for some odd reason) is 6 bits wide, etc.
These were used in 2 ways. Either to store a user keycode (there were 64
keys [1]), or to store a BCD digit. I seem ro rememebr in the latter
case, the 2 extra bits were a sign bit and a blanking flag.
[1] yes, I know the 9100B has 65 keys. 2 of them are wired electrically
in parallel and send the same code. One is 'hyper', the other is one of
the memory transfer functions. If that keyvcode is followed by a trig
function, it's taken as 'hyper'., if it's followed by an address, it's
the memroy function.
-tony