It's complicate. Since the beginning DS were Mk, John, David and Pick. When David left, DS were MK, JI and PW with Alan and Hal under contract. When Pick left, there were Mk and John as DS and all the rest were under contract.
When OES was released, it was agreed that it will be reflected that also Alan and Guy, as oldest members, were part of DS, and the record production was credited to the four of them that way, but DS overseas was formed just by MK and John.
Funnily, Ed Bicknell was featured in DS overseas as well until he left MK in 2000.