That's an unbadged GS...has the oversized AC pulley, final drive should be 3.42, not 3.23...otherwise looks complete. Even has the nice clips for running the battery cable under the oil pan. Whoops, missed those headers. The tailpipes have obviously been changed, but there are shorty headers installed as well. Wonder how it flows now.
The A/C pulley, console shift with bucket seats, and posi are all marks of a 2nd-gen Riviera GS. Only thing missing is the badging and the Star Wars air cleaner...I'm sure there were some unbadged GS models sold to dealers as sleeper specials. I've seen unbadged 65s with the dual-quad 425 nailheads...if they can do it to 1st generations Rivieras, then I'm sure there are others.