IIRC, the M500 is the 'field grade' line and was built all on the same frames. That changed when the "-85" series came out; the skeet sets, for example, were on a slightly smaller frame.
First SKB I bought was a M500 and it served me well for years. The Blue Book shows a 25%+ factor for the 28 ga., so if it fits you OK I'd grab it quick - depending on what they're asking for it.
SKBs are indeed very well-made shotguns, with innards easily compared to any of the Citoris and 725s.
