Many of the Mag Adapters were parts made in Israel and Imported assembled by Mossberg when Mossberg had staated a relationship with IMI to import products but then stomped on by the Clinton Admin. to drop USGI contracts for shotguns if they didn't drop the plan. Many of these would be stamped with UA for Uzi America - what Mossberg intended to operate under.
Some were finished by Mossberg and others sold in parts (Adapter, spring, latch & pin) unfinished and you had to drill a hole for the latch pin and affix the latch - generally it's an easy to do project and what was done by Blackthorn and others.
They generally work with USGI pattern metal mags as was the design of the day.
I had one, used it once gave it to a friend. I paid $25 for the parts and drilled the hoe myself. It worked but I had so many Galil mags that are reasonably priced - why bother?