Pat,
As nearly as I have been able to figure out, over it's production life the Atlas 619 (with 1"-10 spindle nose threads and Timken roller bearings) came with three different countershaft assemblies. The first one, apparently only made a couple of years, was sorta similar to the one on the early (up to 1957) Craftsman 12". The bracket mounted to either the rear of the bed or to the left leg (I have never come across a rear view photo of the original version). The part with the pulleys (that moves to tension the belts) was, judging by later known part numbers, the same as on the two later models. The "A" revision bracket (which you have) is short and mounts to the bench behind the actual lathe, with the countershaft hanger assembly standing up above it. This was used on the 618 for a while and was used on the 101.07301 throughout its production life. The "B" revision bracket was taller, with the same hanger assembly hanging down.
In all three cases, the motor pulley is a small 2-step one. The only changes ever made to this pulley were the change from 1/2" to 5/8" bore and later from Zamak to aluminum. The hanger assembly is apparently the same in all three cases and should have a 4-step cone pulley between the hanger legs which is externally identical to the spindle cone pulley. The large pulley hanging on one end of the actual countershaft spindle is 2-step (outside diameters 7.5" and 9.0"). All of the catalog photos show this pulley as hanging on the left end of the countershaft spindle, and the motor behind the countershaft bracket. A lot of owner photos show the large pulley on the right end of the spindle and some show the motor between the bracket and the lathe.
One other point about which there is a lot of Internet disinformation is that only the 101.07301 had the 1"-8 spindle nose threads. The 618 and later 101.21400 always had 1"-10.
