C500/550 are Multi Pilot aircraft
C501/551 are Single Pilot Aircraft
The difference is minor, something like a transponder ident switch or something ?
Anyway, if it is a C550 it has to be converted to a C551 in order to legally be flown single pilot.
But why fly it single pilot ? I would say, at least have a non-rated or non-current pilot in the right side, if you fly a C501/551.
Or would that happen to be the owner who wants to sit on the first row himself ?