My understanding is that all MAC, MSS, Venoms, Vipers and Thruxtons had rear sprung frames, and all such frames carried numbers prefixed RSS.
Does anyone know if any rear sprung frames were ever made that carried the KDD prefix??
Well, both MAC and MSS were built as rigid machines, later, they made them in swing arm versions, with extensive engine and gearbox changes (not to mention the Swing Arm frame).
VR, VM & Thruxton were only in the RS frame.
The swing arm frames are known as 'RS' because that is the prefix to the frame number. Other swing arm frames were the Scrambler, and late KTT. These are pretty rare though. (There was no RSS prefix as far as I'm aware but I stand to be corrected).
As far as the KDD prefix goes I'm not sure about this, but Velo did make a KDT model which was a modified KTT for speedway events.