The Yale Bicentric cylinder patent, 1,482,189, issued in January, 1924. The mechanism was produced as a padlock, mortise cylinder and a key in knob lockset. I’ve never seen a rim cylinder but am sure they would have made them also.
The purpose was to expand the keying potentials for certain customers who required a MK and more change keys than the G multiplex system could supply. Typically each plug was supplied with a different keyway primarily to prevent the user with a change key from inserting it into a plug it wouldn’t operate. One plug in all locks would simply be KA while the other plug would be a different key.
They were a standard product until around 15 years ago when production was discontinued. They were more pupular on the East coast of the US and used primarily by utility companies. The padlocks appear on Ebay from time to time and I have noticed that most of those are logo locks from OSU.
