Beats me, 29. Had one in .44 mag back in the '70s & every dive shop in The Keys had 'em from .223 to 12 ga - last trip to TKs & you can't find 'em anywhere. BTW, none were ever serial numbered, etc.
A 12 ga bang stick makes a handy all-purpose "removal tool." As far as effectivity, a contact-type 12 ga uses the expanding gases as much as the shot & no never mind about what size shot to use.
If you run across some info as to where to get, I'd sure appreciate a post follow-up. 20/12 ga = doesn't matter.