I have one (and its for sale but not advertised yet) and just love the the pure "grunt" this bike delivers, thanks to the hi compression and hemi- heads ...think they ran squish band pistons ...just an absolute blast to run.
I had a 77 RS as well (before I sold it)...it has the same motor as the S model but I think were 40mm exhaust vs 38mm on the S...I rode both these bikes back to back and the S was definetly quicker ...its a much lighter bike then the RS due to the RS fairing weight....both were lovely machines but the S felt more nimble and quick due in part to the fact it was one of the lightest airheads of its time...
You wont go wrong with a 77 S....
Here's my S ...all original 48,000 kms (about 30,000 miles 'merican) and I have all documentation that goes with it back to the original Bill of Sale in 1977! The original owner, who I knew,was fanatical about maintainece and record keeping. The second owner bought it at 41,000 kms in 1993 and only put on 3,000 kms...

I bought it from him 2 years ago and put 2000 kms on her after a complete retrofit...all original, including the sun faded front fender and rear seat cowl..ha ha...that red paint job was famous for the sun fade..


And the glorious 77 RS ...gone to a good fellow in western Canada (he flew in and rode here back to BC..about 4000 kms)
I put a set of Krausers on for his ride and he sent them back...great guy.

PM me if want to know more...