Fender began using poly in the late 60's. So the '75 MIJ would be period correct with that type of finish, which is also used on the MIA.
Many call the poly finish tone killer. It doesn't let the wood ring out as much because it's very hard and thick. And IMHO, by and large, early 60's Fenders sound and play much, much, better than lates.
The US Vintage series do have a poly sealer under the nitro finish, but the sunburst on my '62 RI P-Bass feels a whole lot thinner than the sunburst on my US Tele. Playing acoustically it sounds alot better than a poly finish bass, too. More natural. Like wood compared to plastic. I believe Custom Shops with a "thin skin" finish spare the poly sealer and have a genuine lacquer finish completely free of polyester.
My primary concern would be the neck. I really wouldn't want a high gloss poly finish on the neck of any guitar I'd be playing. Seems like it would never wear in.
It'd be nice if Fender produced a '66 RI J. 4-bolt, block-inlay neck with Kluson style tuners, and a nitro finish.