The 501 came in two sizes, standard width for utilities like the 8N and similar, and the "long pitman" version for the wider rowcrops. But the basic mower design is identical, other than the width from pitman crank to cutterbar pivot.
There are also several different mounting designs. The common for the 8N type tractors is two anti-sway arms (triangular shaped) which go from the 3 Pt Hitch pins forward to the lower lift arms on the tractor. Older design has through bolts with an upper and lower clamp bracket which grip the tractor lower lift arm when tightened. Newer substitutes a ball and socket arrangement for the through-bolt. Both designs are subject to breakage if not set-up and used correctly.
For larger tractors with Cat 2 3Pt Hitch, there is usually only a single tension arm which connects to the tractor 3pt hitch left lower lift arm, either by clamp or by a through hole and bolt.
In all designs the left antisway or tension arm must be tight as this creates lift for the cutterbar tip when raising the unit.
My previous mowers included a John Deere #5, an Oliver 357.
The 357 was much easier to mount and set-up. There were no clamps or adjustments, just a limiting chain arrangement. Rock guards were slimmer design and less prone to plugging. Cutterbar pitch could be adjusted from the tractor while the machine is moving. However, the 501"s PTO shaft and joints are much "friendlier" to the 8N and other similar small utilities - The Oliver PTO would jamb and chatter when the mower was raised with the PTO running.
CNH does not support many of the parts for the 501, other than knife sections, rock guards, and other wear type items. I broke the outer shoe, to which the grass board attached. Ended up buying a parts machine, which happened to be the long pitman version, which fits my JD 2010RC.
Haven"t used eother 501 in 5 or 6 years. Graduated to a Haybine and will never go back.
I don"t particularly like the 501 design for several reasons: