Today, at my place of work, I went to install a Sargent 6300 core into a storeroom Sargent 10-line lock installed new in 2002. The old core was installed in 2002 as well, and the lock has not had a heavy amount of use. The new core (without the frame around the SARGENT mark on the front top lobe), would go in, the control key would turn back, but I could not remove it. The lever has slight play, so I tried to move it up and down while trying to remove the key with no success. I ended up reinstalling the old core for now, but I am wondering if sometime between 2002 and now, if the Sargent cores have changed enough to make them not installable in those older locks.