4. The unique firearm number must be inscribed indelibly and legibly on a visible place of the firearm’s frame or receiver.
Despite the foregoing, the unique firearm number may be inscribed indelibly and legibly on a place of the frame or receiver that is visible only when the firearm is disassembled if
(1) it is a practice that complies with the practices established by the firearm’s manufacturer;
(2) there is no visible place on the firearm that is appropriate;
(3) the firearm is rare; or
(4) the firearm’s value is exceptionally high for that kind of firearm and such value would be significantly reduced should the unique firearm number be visible when the firearm is not disassembled.