Well, I did some more checking on that Bridgeport No.2 boring head, and I had it all wrong. It didn't make any sense that Bridgeport would make their R8 solid-holder quick-change system with one of the holders not threaded to match their boring head. Just for references, the threads for the Bridgeport® No.2 Boring head are 1-1/4"-20 (not 1-1/2"-20), and the No.1 Boring head is 5/8"-18 threads. This was from a gentleman retired from Bridgeport, but still active in providing parts. He said they made the boring heads. He said the quick-change tooling they made was modeled after the Erickson 30 quick-change, and so far, Kennametal has access to only the R8 spindle adapter part of that system, but no tool holders.
So the upshot is that the quick-change tool holder I have will thread into my No.2 boring head, and I will have an improved system once I remove its R8 shank.
I don't know how I ever got on the idea that the threads I was looking at were 1-1/2"-20. They were right there, plain as day, 1-1/4"-20. Sorry for all the fuss, but I did learn a few things in the process.
Dave
PS: I'd love to find out that there are new solid tool holders out there that with little or no modification will fit into that old Bridgeport line of QC tooling. It is a real fast and solid system.