I was talking to a friend and he mentioned that there was interest in these with a 1500 serial number or below. Is it because the different padded case or am I missing something?

I think the case was different, and also the early ones just had the serial number and proof marks on the receiver, while later ones also added that info to the lower.  Those differences are only worth something to people who think it does.  Personally, I don’t think it’s important enough to seek out, or pay a premium, for an early serial number.

I may be incorrect here, but I believe later serial numbers have the diopter rear sight already included.

That’s true, they came with the castle rear sight until recently.  That was a very late change, maybe in 2021 production.  Mine has a serial number over 4000, and still came with the castle sight.