I personally don't feel it would be impolite at all. Hey if I'm putting down my money, I would like to know everything I can about the firearm. Especially if I am buying it as a military collectable, which has a direct impact on it's value whether or not the "numbers match" (if there are any to match). Not talking specifically Enfields right now.
A direction that may taken if the seller/owner does not wish the dissassembly of the rifle; if I buy the rifle and find that it does not match the seller's description, ie. matching numbers, then can I bring it back for a return of money. That way I've given the present owner the cash, and if I in turn wish to examine the rifle by taking it apart, I live with the consiquences. And also in turn, if it isn't what he is purporting, he takes it back.
It is lousy buying a "pig in a poke", and I'm not that rich to take too many chances.
Besides, it may also keep the "embellishment" down a little on the description, and a buyer can gauge a seller a lot through his willingness to stand up or guarantee his firearm. Therefore might be a good indication of the worthwhileness (is that a word?) of buying that particular firearm.
Just my opinion after buying 2 or 3 military surplus type firearms.
P.S. There are lots of LE informed guys right here as a resource.