Guitars actually built earlier in the run (aka, shipped in 68) tend to exhibit routes with straight walls and a flat maple floor.
Late 1960's Les Paul Customs were only available in Black.
The primary difference lies in the body construction.
Simply, LSLP have maple tops, whereas 50's examples had solid mahogany bodies.
Early “long tenoned” examples bring the most money and are accurate to 1950 specs.
(Although, 1950's guitars have a rounded tenon end vs.       They were followed by what I refer to as a “transitional tenon”.