I agree it is probably for a Hi-Standard .22 Mag derringer.
I think it is a one-of, and probably made by a leather craftsman with no knowledge of Federal Firearms laws.
I know you know, but for others reading this who don't know!
Be aware that any type of shoot-through holster wallet that completely covers the gun and is not "readily discernible as a weapon" is illegal under Federal Law.
If you put a gun in it, the gun & holster become an AOW, or "Any Other Weapon" and would require a tax stamp & registration from the ATF.
rc