can't comment on the disc mower.. not sure what size it is.. not familiar with that model.
a 6610 was made from82-93, 4 cyl diesel, 268ci, i see listings of 78-82 engine hp, and a pto listing of 72 pto hp. cat 2 lift, independent pto.. etc.
the 6610s was a mexican made unit, 94-2003, 4 cyl diesel, 304ci , about 85 engine hp and 80 pto hp.
I have a 7610s, it's big brother. when i got mine a year used from the dealer, it had a pretty big disc mower hooked to the back of it, and it's about 95 eng hp and 87-90 on the pto depending on who's #'s you use .
somewhere before the start of the S series, towards end of the 10.. you got the II series.. etc..