Hello. That vintage Pre-B is probably not going to be a short-railed version which would be worth more from a collecting point of view. It's probably the standard Pre-B similar to mine.
I think the price is a bit high for a Pre-B that's not like new.
$400 is about as high as I'd go. On the other hand, these guns are getting harder to find with each passing year. Understand that some of the internal parts are not the same as on the 75B pistols and that aftermarket magazines that fit the B might not fit the Pre-B with its very slightyly smaller magazine well dimensions.
Hello. I really don't think there's much of a chance if it was made in the 80's but a most obvious way is to look for the lightning cuts on the front of the slide to extend back perhaps twice as much as on the "regular" Pre-B's.