Cudak888, thanks for the serial number info; I did find the number on the left dropout. The bike is #50680064, with no letter prefix. I think this could be a 1971, although 1967 and 1969 are also possibilities.
Tzigane, that's a great Sturmey Archer link.
KO57, thanks for posting this thread. Nice to have some company in appreciating these Columbias.