I met up with my friend I got the gun from today. He gave me a bunch of ammo (mostly Hornady Critical Defense) and an older holster. It's marked "Longhorn" on the bottom of the back, thumb snap. I found a holster company with that name in Massachusetts that opened in 1965 and is still in business. This is definitely a period style piece as the gun is from 1978.

I think I'll be shopping for an open top pancake style in the very near future.