The Berlina, Statesman, Calais & SS are built on the same frames. SS is slightly different in the padding. These "upper level" frames & foams are easily distinguished by the recessed back and pocket(where fitted).
Caprice has electric ones built on "upper level" frames (similar frame & padding to stato)
Exec, Acclaim, S, Equipe, esteem, etc all share the "basic" or "low spec" frame & padding - you can pick it by the flat rears that don't recess in where a pocket would go.
The basic HSV seats (Langtons etc) are a step above the SS - like the manta, clubby etc ones, but are built off the same frames as the "upper" seats.
Then you have the top-line Devenish HSV seats as used in the senator, GTS, GTS/R etc with the chunky wings (and associated bracing on the framework under the foam) that are built on unique frames - which are modded "upper" level ones.