There are in fact two different selector arms use among the five different QCGB;s that Atlas made. However, this is the first indication I've seen that they were different lengths. We knew that Atlas 10-1506 Selector Arm was used on the Atlas 1500 (and 1570, which differed only in having a template for drilling a hole in the 10D and earlier headstocks). And 10-1586 was used on the later Atlas 6800 and all three of the QDGB's for the various 12" machines. We know or at least assume from the design that 10-1506 was Zamak and that 10-1586 was cast iron. There also exists a photo in the first parts manual on the Craftsman 12" QCGB that shows what can only be a Zamak part. But the same document also shows the arm as being 10-1586 and the drawing as opposed to the photograph shows the flat sided cast iron version.
Most of the parts, other than the main castings, are the same in all five versions (or seven, if you count the parallel Atlas part numbers for the two versions used on the Atlas model numbers of the later 12" lathes). This would tend to imply that the two arm part numbers are physically interchangeable.