If the collets are 3AT or 3C, 1/2" is the largest size that either of those types come in, and is the largest size that will fit a 9", 10" or 12" Atlas or Atlas-Craftsman lathe, because that's about the limit on the draw tube diameter, which has to be somewhat larger OD and ID than the collet capacity. And the collet closer adapter must also fit into a 3MT taper.
If the collets are ER type, there is an upper and a lower limit for each ER size.
One would be hard pressed to differentiate between 3AT and 3C from just looking at them. But 3AT or 3C should be engraved into the front face of each collet, along with the size. If you ever end up with a mix of both types, the 3C is about 3/8" longer. There are some other differences but that's the only one that's obvious.
I found another chart on collet size length, dia, etc and it makes me question which ones I have , thread looks to be 26tpi my thread gage has 24 tpi but is not exact fit the thread dia is .690 oal is 2.62 back bearing dia is .702
OK I figured it out they are stark 32S 3 1/2 collets 24tpi threads made for the stark lathe with the closer and the draw tube. here is the page I found https://xtronics.com/wiki/Collet_Dimensions.html I have used a couple of them and they work very good in my atlas 10F.
OK. I thought that I had just about every collet listing in captivity but this one has several collets that aren't on any of the lists that I had. Thanks. I'm glad that that question is now settled.
In case you haven't figured out what the columns headed Round, Hexagon and Square are, those are the largest size for each of those shapes that collets were made for.
I do notice that one dimension that could be critical in ID'ing an unknown collet closer adapter is not on the chart, and that is the collet head taper angle. Also, as long as the list is, it includes no metric collets.