You have to remember that back in the day when this was probably made, there was no internet. The phone company charged for long distance calls. If you ordered something, it was done by snail mail to and from. If it wasn't in the case at your local Mom & Pop gunshop, or you managed to run across it at one of the two or three gunshows there were a year, you weren't going to find it. There was virtually no way to know anyone had one for sale unless you ran across it yourself, someone else ran across one and told you, or you saw an ad in the newspaper.
So alot of people just had what they wanted made. It seems crazy now, but back in the day things like this were often the easiet way to get what you wanted.
Think of it as a very localized market. In one part of the country, there were HD's everywhere because that's what had been popular in the past and used ones changed hands more often. In another part of the country, you'd live your whole life and never see one outside a catalog. It wasn't like today where I could read great information from someone in PA, MT, TX, CA, KS, FL, etc. within seconds. Or know what someone in PA, MT, TX, CA, KS, FL, etc. have for sale. I'd never know.
As far as the gun goes, I'd have to agree that there is no collector value. Custom work rarely adds value, and it's a rebule. If it was done years ago when this sort of thing was more common, it was refinished after the work to begin with. So it's quite possible it's on it's third finish. I wouldn't pay big money for it. The only way I'd pay as much as a reblued 28 would be if I was able to inspect it in person. I personally like it. I'd buy it if it were cheap enough, but not unless I could see it in person. Once you start talking about guns that are modified, you really should look at the gun in person unless you know who did the mods and are otherwise comfortable with them.