Do you know of any other differences?
Some books for your library that contain at least some MSS content:
GeneralAllen, C.E. 'Titch', Velocette Saga, The, Amulree,1994
Bacon, Roy, Velocette Viper, Venom and Thruxton, Niton Publishing, 1990
Burgess, R.W. and J.R. Clew, Always in the Picture: A History of the Velocette Motorcycle, Haynes Publishing, 1980
Burris, Rod, Velocette: A Development History of the MSS, Venom, Viper, Thruxton and Scrambler Models, Haynes Publishing, 1982
Henshaw, Peter, Essential Buyer's Guide, The: Velocette 350 & 500 Singles, Veloce Publishing 2016
Main-Smith, Bruce, First Velocette Scene, The, Bruce Main-Smith, 1977
Masters, Dave, Velocette: A List of Models, D.J. Masters, 1974
Masters, Dave Velocette 1905-1971: An Illustrated Reference, Transport Bookman, 1976
Moseley, Leonard J., My Velocette Days, Transport Bookman, 1974
Rhodes, Ivan, Velocette: Technical Excellence Exemplified, Osprey Publishing, 1990
Walker, Mick, Velocette Production Motorcycles, Crowood Press, 2006
ManualsClew, Jeff, Owner's Workshop Manual: Velocette Singles 349 and 499cc, 1953-71, Haynes, 1975
Leigh, Ferrers, Book of the Velocette, The, Sir Isaac Pitman & Sons, 1970
Lund, Gordon, Velocette KSS Mk 2: A Restoration Guide, Brooklands Books, 2018
Main-Smith, Bruce, Velocette Viper/Venom/Thruxton 350 and 500 Singles, All Years, Bruce Main-Smith, 1978