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Good deal. I had a similar situation with finding a wr 565 cassette deck service manual posted on-line. All I found was a pdf file for I think it was the wr 535 and 545 models. Thankfully, it seems the Sony 500 series of dual cassette decks are all constructed similarly. It would make economic sense that the models might share the same specs. in a assembly line pragmatic sense.
If it's similar in construct to a Sony tc wr565 you'll have four small Phillips head screws in each rear corner of tape transport - you'll need a small screw driver for this - as well as removing - on the tc wr565 any way - three Phillips head screws that affix the transport halves together. I jumped ahead of myself. Prior to backing out the four corner screws, have the transport cassette door open with the front door panel removed - probably slides up. This will allow you to back the transport assembly door and all out the backend of the tape well mount hole. Of course make sure to first disconnect any cables that attach to the circuit board from the transport assembly. Probably flat prong plug cables or harness type. Take your time.
I confirm about the mount posts - referred as 'claws' on the disassembly schematic.I found the transfer of the belt, from the claw, to small white plastic pulley to occur pretty naturally as halves snapped back together. The smaller, I think it's the capstan belt, took a little bit of manual assistance on my part - aided by the flat blade of a millimeter size tech screwdriver, (hint: have a set handy) to get it seated on it's small plastic pulley. Overall, straight forward operation. Took approximately an hour. The decks back to working like it did when I first bought it twenty odd years ago. Only set me back twenty five bucks for a set of replacement belts. I'm happy.
I bought some #64 rubber bands at the local office supply. It didn't work for my Sony tc wr565 cassette deck. It might of worked - possibly with a smaller size rubber band. I ended up just buying a replacement drive belt kit for twenty five bucks. That solved the problem. An unseen benefit, however - I now have enough #64 rubber bands to last me, probably, about five life-times.