Norton did not produce a model 650ss in 1968. I think 1967 was the last year.
The Mercury was introduced in 1968. The cases on a Mercury are stamped 650SS, but the model was not designated as such.
Some Mercury's owners added the chrome and paint of a 650SS.
Perhaps your bike is a Mercury?
Below is from the NOC.
Stephen Hill
"The Norton Mercury was made, and dispatched, from engine and frame numbers129146 to the last one being 129896. They were dispatched in the years,1968,1969 and in 1970."