I found the Customs law on the parts being banned from import:
12.95 Definitions.
Terms as used in §§ 12.96 through 12.103 of this part are defined as follows:
(a) Switchblade knife. . . . any imported knife, . . . including Balisong,
butterfly . . . knives, which has one or more of the following
characteristics or identities:
(1) A blade which opens automatically by hand pressure applied to
a button or device in the handle of the knife, or any knife with a
blade which opens automatically by operation of inertia, gravity,
or both;
(2) Knives which, by insignificant preliminary preparation, as described
in paragraph (b) of this section, can be altered or converted
so as to open automatically by hand pressure applied to
a button or device in the handle of the knife or by operation of
inertia, gravity, or both;
(3) Unassembled knife kits or knife handles without blades which,
when fully assembled with added blades, springs, or other
parts, are knives which open automatically by hand pressure
applied to a button or device in the handle of the knife or by operation
of inertia, gravity, or both; or
(4) Knives with a detachable blade that is propelled by a springoperated
mechanism, and components thereof.