The reason there's such a mix of applied standards is that to fall under the "antique - not a firearm" rule it has to be a true antique, made before 1898, and you don't know that with a Khyber pass special. Add to that that late Martini rifles were chambered in 303 British (which you can still get at any better gun shop), and you're in violation of pretty much any gun rule there is if you mail them home. Approval from the US Customs office won't help there, they're only interested in monetary value type things. You'd need an AFT letter stating that the weapons indeed fall under the antique rule first.
F 135 - the right choice