And the answer is -- None Of The Above!
The holster you guys are thinking of is one that was originally designed for the Hi-Standard double derringer, which made it appear to be a wallet. It had a hole in the center through which the trigger could be pulled, firing the weapon.
A wallet holster is perfectly legal, so long as the weapon cannot be fired inside the holster. I made one for my P-32 which worked fine, until I put a pocket-slipper laser on it, which eliminated the need for the leather holster.
ATF has previously determined that firearms installed in various
types of wallets, briefcases, canes, etc. may be NFA firearms as
defined. The submitted samples are rectangular leather cases,
which measure approximately 5 1/2 inches in length and 2 3/4 inches
in height. They are designed to hold an American Derringer firearm
(Model DA 38) in a fixed position with openings in the leather so
that the derringer can be operated and fired while it is in the
case. When the derringer is installed in the case, the weapon has
the exterior appearance to other "wallet guns" which have
previously been determined to be NFA firearms.
I was referring to the holster in which the pistol could be fired from, which is an NFA item.