[DougMerritt]

IIRC I think six characters was also the maximum for the length of global symbols in C on early Unix systems, possibly just because that's what everyone was used to on earlier systems.

But note that I asked about why six characters, not why six bits per character -- however your note is perhaps suggestive -- maybe the six character limit is similar to the six bit character after all: something established (possibly for mechanical reasons) in 1928? Perhaps?

[lapsed_lisper]

Right, good questions. Pure conjecture on my part: maybe it's just that 36 is the smallest integral multiple of 6 that also had enough bits to represent integers of the desired width?