FWIW, during the last 2-3 years of LE production for 3rd gen's, when agencies ordered full-size duty weapons they were often offered the option of having them produced with riveted or integral rails, at no price difference.
I only found this out when talking with the guy who was responsible for placing our orders for our assorted TSW's back then. While our first shipment of 4566TSW's had been with the standard riveted rails, when our next order went in the factory asked him if we wanted the guns made with the integral frame rails, for the same price. (We could've ordered them with the Melonite hardening treatment, too, for a slight added cost per unit.)
He said he'd told them to stay with the riveted rails for our next batch, as he didn't want anyone who had received the earlier riveted rail guns to feel jealous of anyone getting integral railed guns.
Really? I asked him why the hell he'd not discussed it with anyone else, and reminded him that the integral railed guns couldn't suffer the same sort of impact damage the riveted rail guns occasionally experienced. We had a few guns with broken rails from being dropped.
Sigh.
If that second half shipment of 4566TSW's had been received with the integral machined rail, I might've been tempted to buy one as a former duty weapon when that was offered to us as the TSW's were removed from service (replaced with plastic). Just because.
I don't see the integral railed guns as being especially rare, because all 3 calibers of the full-sized TSW's were being offered to agencies during the last few years of TSW production. Granted, this was after the 3rd gen's had been dropped from the commercial catalog, so the TSW's being produced that way had primarily been for LE agency orders. Of course, the way S&W had typically done production runs, some extra units were often made when special orders were done, which ended up being made available for individual officer sales, agency 'last minute extras' ... and the occasional release of overruns guns through one of the company's major vendors.
The Melonite guns were usually the result of special orders for state, local or fed agency orders, or a PC project, or guns previously shipped for international sales and then imported back into the US (think Modelo Militar or Mexican Special Forces 5906, etc).