I finally had a chance to work on the 550. There is no clutch chatter and it appears to have some adjustment left. I took out the shifter and it seemed as if all the rails were lined up. I can shift it into each gear, or feels like it does. In one position releasing the clutch will stall the engine if I were to let it, tractor does not move. I can get what I think is first gear and the tractor will go forward. Any other selection and I get nothing. So I take the top off the transmission for a look see. Before I did, I removed the plugs to drain the gearbox. It was already empty. My thoughts that someone had been in before me have been confirmed. I did get a few pieces of metal off the plug magnets, sure looked like teeth fragments. Once I had the cover off I don't see any damaged gears. What I did find is the bearing at the center casting has weld around it and appears that two threaded shafts are welded together. I've not been in a transmission before on a 550, but I don't remember seeing anything like this when I had the 1800's gearbox exposed. I've attached a couple of pictures. 

I think I need a transmission at this point... what say the experts?