Looks like a 60s 500/650 twin seat with side hinge. Later oil in frame models may use the same/similar design though. Someone like @darkman may know better!
After looking at seats on Baxter's website, it appears to be a T120 1967-69. The bolt holes are parallel to the sides. The two bolt holes on a late 70s triumph are running perpendicular to the edge.
More info 1967 650 pan never had the pressing/resses right at the front as 67 pan was a one year only pan Also 69 only had 4 extra nuts for rear grab rail so its not 69 either lol.
