Quoting from To be recognized as C&R items, 478.11 specifies that firearms must fall within one of the following categories:
- Firearms which were manufactured at least 50 years prior to the current date, but not including replicas of such firearms;
- Firearms which are certified by the curator of a municipal, State, or Federal museum which exhibits firearms to be curios or relics of museum interest; and
- Any other firearms which derive a substantial part of their monetary value from the fact that they are novel, rare, bizarre, or because of their association with some historical figure, period, or event.
So, a starting point would be to explain why the P-83 Wanad pistol has historical interest (that is, define why they may be "of special interest to collectors by reason of some quality other than is associated with firearms intended for sporting use or as offensive or defensive weapons").
The P-83 isn't 50 years old yet. Can you get a curator to write to the ATF explaining why it may be of museum interest? And they are not really novel, rare or bizarre (though one could maybe make a weak argument that they are associated with the Cold War, but a hundred other firearms fall under that argument too, so it might be a challenge grounding an argument that a substantial part of the P-83s monetary value lay in it's association with a historical period or figure).
Not being argumentative ... bureaucracies follow rules and regulations ... to make an argument, you'd need to figure out how the firearm clearly and unambiguously fits any of those criteria. (Bureaucracies also can have their own logic - what is on the C&R list is an interesting read).