John,
I believe the latter SL s had Aluminium Hard Tops, so may be lighter, and had less glass. Ours are steel and do weigh 50 Kg.
I load tested my lifting kit using 30 full "Coke" type 2 litre Bottles.
As others have said there is a multitude of information here, of various lifting designs.
Personally, I do not like the "Gutter Rail" type, so I use two "L" type bars under the side windows, which I modified from a German Audi system..
Garry is the expert though !!!!!!
Happy lifting
Paul