For a truly collector piece, you would not want any modifications, including to the original stocks, but you would probably also want it in factory NIB condition. I expect the overwhelming majority of revolvers that had a metal plate added to the bottom of the stocks spent their lives riding in a duty holster and got shot at least twice a year, if not more often. That means they are not collector items for the purist. If the revolver was stamped so as to indicate which agency purchased it, it may have some extra value to someone who collects former law enforcement firearms.
Given that, unless it is shot to death and needs significant work to make it a good shooter, a Model 66 of any variant for $300 today is an incredibly great deal.