Don't know too much about these bikes, but I believe only minor details separated the look of the 18S and G80S.
My '61 G80CS has quite a few differences from the '61 'S' - smaller fuel tank, alloy fenders, QD lights, different seat, different rims and tires (front wheel was 19 or 21" dependent on whether scrambles or trials, I believe the 'S' wheel was 18"), handlebar, footpegs, etc. Magneto is also a competition one, with cable advance/retard. I don't know how different they were back in the mid-50s, but I'm sure you can look at brochures over on Christian's archive.
A word of warning. The front brake anchor on these bikes has a very nasty habit of snapping through the bolt hole where it bolts onto the brake plate. Inspect carefully and dont use if it is worn or chromed !!
I had a 1953 G3LS as my first bike at age 15. They are a nice old machine.