I say '57. That usually does it for the casual types. If questions keep coming, and there is interest in the car (and me) I go in to the details of the replica build. So I amend my response to a '72 -- the donor V-dub. Then as I explain about how much of the car is brand new, I end up saying it is an '07.
I won't say this is exclusively the only question asked, but it is by far the most frequent. Seems to me that someone is either going to know what the car is, or he is not. If he does, then he will not ask the year question (because he knows the answer well enough), but something more intelligent, or maybe just exclaim how beautiful it is, etc. etc. -- all too true. If he does not know what the car is, and he is standing there next to it, he can see plainly that it is a Porsche (says so front and back) and so he is stumped, 'cause it sure don't look like any Porsche he has ever seen. So he has to pull on that thread, but recognizes his ignorance of the brand, and can only wonder what year it must be. It's how it works for me, actually. When I go to a car show and there are old or obscure brands there thast I do not know ver4y much about (e.g., Alpha-Rs), while admiring these beauties, I wonder what year they were built. It's just natural, for some reason.