I just went thru this issue with my 49 GMC two ton Coe. Has the SM 420 with an unknown PTO unit on it. I recently tried to replace the worn unit. Brought it to my local Clelses dealer who called the factory. Factory could build one to replace it. About $500. Get the tranny numbers and forward them to the Chelsea Dana type suppliers that can be found on the internet. I`m sure you can find a rebuilt or new unit.