The PDP-10 and the GE/Honeywell were the two machines I recall that elicited Dennis' comment about 10-track tape drives.  When I ported C to the Honeywell machine at the Murray Hill comp center, I used 9-bit bytes as the default, and added a syntax `abcd` to create a constant in the 6-bit character set.  Most of the OS calls used 6-bit characters, although the time-sharing system was moving to 9-bits.  And most of the use of C on the Honeywell was in the time-sharing system.

Quite a few years later, I discovered accidentally that the syntax `abcd` was still accepted on the Sun compiler, that had been based on PCC.  It drew some kind of error message like "GCOS characters not supported", presumably because some switch was turned off in the machine-dependent files...

Steve