Yes about the 5200, this is the order of first to last and small to big. The cylinders and pistons will all interchange between them. Might have to a little filling on the fins to make things to fit. The 8500 has a bigger fin area than the 5200.
4200 68cc 5200 85cc
Starter has 4 10- 24 screws holding it on and the rear handle doesn't have a step for your foot to start. The cylinder cover has either a center screw or 3 screws to hold it on.
4400 68cc 4900 77cc 5400 85cc
The Starter is held on by 4 1/4- 20 socket head screws. The fuel tank bas been extended by a inch and a step is added. Cover for the cylinder is held on by 3 screws.
6900 68cc 7700 77cc. 8500 85cc
The big change is the cylinder cover and air filter, the back part comes off without having to take the cover off. Air filter is a nice screen type. On the inside the crankshaft has been beefed up on the output side and left at 1/2" from the other ones that are 7/16".
Can't think of anything else other than the 68cc pistons are thick ring and the other ones are thin ring.
My feelings are that if you find one you should send it to me because you don't want it. Enjoy your saws.