I would have to see numbers but the 5" version of the PPC with the scalloped or bullnose type slide was the first version to come out and was gobbled up by PPC shooters , within 2 years it was the gun that dominated ( in numbers) the line at most matches. The 6" version ( with the same bullnose ( for lack of a better term) came next from S/W but only to the very top shooters, or those that had good connections. This is by far the rarest version, of the PPC model. Im not sure it even was offered for sale, Ive never tried to hunt one down , I think most of the guys who had them, sold them off. The second version of the 5" gun I would think is next rarest , because guys who had a 5" already didn't need one and the 6" came out at the same time. Again just my thoughts as what I saw on the range at PPC matches.
Then the 6" version came out with the full squared off slide. I always thought the term dustcover referred to the front portion of the frame, but I hear it used both ways frame and slide. I thing the frames are all pretty much the same.
Well not that many PPC competitors jumped on the 6" gun because it was only 1" longer, and you couldn't shoot the Distinguished match with it. About this time the longslide 1911s were starting to appear from some of the custom makers like Tanaka, Clark, Springfield etc. Many competitors didn't like shooting autos anyway, or only did it for distinguished matches and the Nationals so couldn't justify another auto purchase.
Then the NRA changed the rules for the Distinguished match making 6"guns legal and guys that previously hadn't wanted one scrambled to get a 6" S/W but by then the P/C was slowing down with PPC and had a lot of other stuff coming.
If I have the story right, and I spent quite a bit of time(socially) with the guys from S/W, Jim Rae and Tom Kelly. These PPC autos were actually made for European distributors. Like Lou Horton/RSR etc, in the US. The distributor allowed S/W to sell the models (Target Champion) with Aristocrat sights to LE only , because S/W saw a market with the beginning of S/A matches in NRA Police Combat, at least that's the story I remember and the distributors wouldn't allow sale in the civilian market. Also some legal worries about the removal of safeties on these models.
The first picture shows most frame slide variations. Second photo has the first version with the scalloped nose/slide.
Also a 4" and 5" service auto version of the 5906.