Remember a few years ago someone here that has a large Morgan collection. They didn't realize that Morgan 150's exist. Here's one on ebay.
https://www.ebay.com/itm/MORGAN-CHICAGO-150-Vintage-MACHINIST-VISE-On-Custom-Stand-Local-Pickup-ONLY/163368646211?hash=item260987b643:g:A2UAAOSwBMpb6MnE
I have a 3-1/2" Starrett 643-1/2. Not quite the smallest I think.
http://www.garagejournal.com/forum/showpost.php?p=2864407&postcount=7199
Finally realized the swivel pin is threaded in. You just unscrew it.
The collar is mild steel. You can reposition to collar several times and use a fresh unbent tab. You could also fab one of these with some tubing and hand filing.
My Prentiss 20 has a set screw locking collar made out of some thin tubing with a set screw. A normal thickness collar won't...
The original Prentiss collars have fingers that are pushed into a slot. Might need to spin the collar and push a different finger into the slot. Or convert to a locking collar with a set screw. But there probably isn't much clearance so you probably need a custom thin walled collar.