Shell holder differences?

Wise folks of the forum, what might I have here?

Backstory:
Years ago, I vaguely recall a bit of difficulty in getting this shell holder into my RCBS single stage press.
Once before I wanted to change calibers to reload, but that shellholder would not come out.  This time I was stubborn and after only 4 or 5 hits with a ball peen hammer, it broke loose.  That was after being soaked in Kroil for a week and I should add the shell holder and ram are shiny... no rust.

I'm looking at the #2 marked shell holder from my press and comparing it to a Lee #2 shell holder:
I can visually see that the height of the lower "rim" is different.  The mystery one (which I undoubtedly picked up at a gun show some years ago) is 0.124 inches, the Lee is 0.114.  Additionally, if I turn the shell holder over, it looks like the crown of a barrel, whereas the Lee one is machined flat. The Lee shell holder easily slips in and out of my RCBS press, so that is one I'm using from now on.

So, I thought these shell holders were all standardized, but obviously not.  What gives?

Additionally, I see that Lee recommends #2 for reloading 45acp, yet RCBS recommends their #3 shell holder for 45acp.  Once again, huh?

Thanks for 'xplaning this to me!

Greetings,

Some of the old loading books have charts listing the various manufacturers and numbering system. The Speer books come to mind.

Cheers,

Dave

Grafs and sons has a online pdf cross reference