Notes: "The K&E Model 9068 Polyphase Duplex slide rule first appeared in 1949, in a price list for the 1944 K&E catalog, not in a new catalog. The 9068 model number did not last long; by 1952 it was listed as the “4168 (formerly 9068)” in a K&E educational products catalog. So, dating this rule is sorted out. The 4168 slide rule was made all the way up to 1972, although by then it’s model number had changed once again."
