Hi Ron,
We are in vehement agreement <img border="0" alt="[grrrrrrr]" title="" src="graemlins/cussing.gif" /> but let me clarify my context a little...
In this particular case (a swedish mauser rifle cutaway) the receiver and frame would be the same part. But if we were dealing with an autopistol they would likely be different parts.
If the controlled Item (the serial numbered part) were able to allow the discharge of a cartridge by the substitution of parts, then I believe the item would be correctly classified as a firearm and not a Non-Gun.
If the serial numbered part could not be made to fire a cartridge regardless of how many parts were replaced, then I think it could correctly be classified as a Non-Gun... of course ATF and Customs inspectors would have the discretion to classify the item as they see fit and unless one is prepared for legal action and the associated costs, I doubt you will get them to change their minds.
I have regular professional exposure to both of these agencies...especially U.S. Customs (which just got renamed to be the Bureau of Customs and Border Protection under the new Department of Homeland Security) and they are a tough crowd to get to change their minds once they have made them up, but they are also reasonable when circumstances permit it.
They have to err on the side of protecting our turf from unscrupulous smugglers... and generally they do a great job... unfortunately, because of the threat of smuggling, ordinary citizens can sometimes get burned over technicalities like this situation.