Trying to figure out what I have here

[ShagDog]

Well, I am happy to report that the chuck came off with a little persuasion. I gave it a couple doses of Kroil and let it set thru dinner, tonight. I was able to break it loose with a long piece of lumber and a good pull. I must have had it off before, as there was some anti-seize on the threads, which looks like something that I would have done.

The back end of the spindle measured out at about 1.4” ID. The thread measured at 2-1/4-8 on the chuck end. How can I tell if it is designed for use with collets? I am not familiar with what I would need to use collets. For now, I am going to look for a 3-jaw scroll chuck.

Yes, it will take collets (see previous publication I linked earlier in this thread for the 2.25" thread spindle http://vintagemachinery.org/pubs/1617/16606.pdf).
You will probably have to pay quite a bit for a used set. As to collet info, you may want to check out this old South Bend catalog starting around page 48. http://vintagemachinery.org/pubs/1617/4569.pdf

A