I'm born and raised Canadian, live on the north side of Lake Erie.
Just outside a small, and I mean small town called Lyons. The closest major center is London and its 25 min.'s or so NW. This is farm 
country here, corn, soy and cattle as far as you can see.

You are not the first to mention that they have never seen a cab
on one before, I trust you followed the link to the picture.
The cab serial number is 11.506 and its a model "tractor 2-44". based on the model and the fact that it just fits the
loader frame and tractor so nicely I believe it was built specifically for the 550, I will try to get a manufacture name 
for you

On my 2-44 I'm not real happy with the power steering circuit. Its
not the original pump (I think) as it just dose not seem to sit correctly, the mounting and adjustment brackets look like they have
had some after the fact hacking done to them. My brother-in-law is a mechanic for one of the big harvesting companies in this area and 
he suggested that I run the power steering circuit off of the existing loader system. Now I'm a Mechanical Engineer and work with 
hydraulic systems from a design standpoint all the time so I got thinking about his idea and if I run my pressure line to a flow control valve
to divert off some flow and then through a relief valve to insure the PSI in the steering system never gets to high it should work. At worst 
it may slow down the loader speed which seems very fast to start with.

Have you ever heard of this being done, it must be somewhat common.

Ron