I looked at the Sams manual and it doesn't seem to show significant differences amongst the various chassis listed. All use a 21-inch picture tube. A few chassis use a different type of tube for the sync separator. A few other minor differences are noted in the parts list.
I would look for all of the listed model numbers, and if you're lucky enough to find any of them, worry about small differences later. As long as the chassis and picture tube physically fit the openings, and the chassis has the right knob shafts in the right places, you don't really care whether it uses a 6xyz or 6abc sync separator tube.
As Electronic M noted, it was common to use the same chassis in multiple cabinets. The manual for my RCA CTC-11 lists 43 (!) model numbers. They all used basically the same chassis, with two electronic options (UHF tuner, remote) and a zillion choices of cabinet and trim.
You may need to be patient when looking for a specific item like this. There are no Wal-Marts with hundreds of old TV chassis to choose from. It might be easier to find another complete Admiral with a junky cabinet and a restorable chassis.
Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html