Mick,
The Penton pic you posted does appear to have 78 forks & triples. The swingarm appears 77 (or modified back to 77 spec). The tank may just be sitting further forward w the slop in the system.
Its hard to know about the frame tho. The top shock mount on the 78s was quite a bit further forward & up compared to 77's (at least the early 77s), but that one seems to be between the 77s & 78s. Maybe it's late 77 frame
Or a mod'd 78 frame.
The footpeg location is diff too. Looks like they've been moved forward like the 78 MC had. (78 GS was more rearward, like 77 GS & MC))
I believe the 77 & 78 GS ran the hi-breather frames w pressed steel backbones. If yr worried it maybe GS frame, ask for a pic w'out the tank. A pic w'out the sidecover may reveal a bit too.