I read this a few months ago and just happened to save it:
As defined in 26 U.S.C. section 5845(e) of the National Firearms
Act (NFA), the term "any other weapon" includes certain weapons
capable of being concealed on the person. It is unlawful to make,
possess or transfer such weapons which are not registered in
accordance with NFA controls. Further, a maker of such firearms
who has not paid the appropriate special (occupational) tax is
liable for $200 making tax for each weapon produced and $5 for each
weapon transferred.
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
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.
ATF has previously determined that an American Derringer, Model DA,
in and of itself, is a firearm subject to the provisions of the
GCA and is not a firearm subject to the NFA controls. The leather
wallet by itself is not subject to control as a firearm. However,
we have also determined that any person who might possess such a
derringer in combination with the wallet holster would be in
possession of a firearm subject to the purview of the NFA.