The 501 was sold as "Citation ISP", SP meaning single pilot. This was a C500 airframe with a different AOM (and boommike, A/P), but same weights etc.
(501 was far 23, the C500 far 25)
Same story basically for the II:
The 551 is the Citation IISP and is essentially the same airframe, but MTOM was lowered from 13300lbs to 12500lbs. So was MZFM and MLM, but I donīt recall the figures.
Never came across a C561. C560 was always a 2 man crew a/c at least in germany. (in the US it is still SP
AFAIK)
Before JAA the typerating was C500/501/550/551/560.
Now there is a CLASSRATING C501/C551 and a TYPERATING C500/550/560.