This transmission is used, at least, in the John Deere 200, 208, 210, 212, 214 and 216. May be other models and brands I don't know.
I encountered some problems with my 214, I will explain those and the causes/solutions.
First sometimes the transmission locked itself... in this situation if you release the clutch the engine will die as the transmission is locked! Over time I found that the problem presented itself only if I was using 3rd or 4th gear. Also at this point if you put the shifter arm to neutral, the tractor will be able to move but only in the gear that got engaged, most of the time 2 or R. You will NOT be able to return the R-N-2-1 to neutral! If you get in this situation, a temporary fix is to remove the shifter arm from the transmission (after removing the %$#?%#$? fender deck), move the fork to align it with the 3-4 fork and reinstall. At this point use only R-N-2-1.
The problem originates in the transmission. There are 2 forks, one of them moving a gear between 4 positions to make the R-N-2-1 ratios. In both forks there is a ball pushed by a spring that engages on the fork's shaft to "lock" it in the selected position. The spring was broken in the fork. When the transmission is selected to 3rd or 4th, nothing prevents the R-N-2-1 fork/gear from moving due to the tractor angle and/or vibration. If it moves, you will have 2 gears selected! Also, if it moves, you will be forever locked in the gear.
This problem may appear "opposite" to that, if the 3-N-4 fork spring breaks, the 3rd or 4th may select itself!
The solution is to replace the spring M40350, they are inexpensive. Don't be as stupid as I was, replace both springs. Might be a good time to replace both outer axle bearings!
Next problem began after replacing the spring and might be specific to JD tractors. The 2nd gear was unselecting itself at random times. The problem is that the R-N-2-1 fork's ball cannot reach the groove in it's shaft.
The solution is easy (but I found it after removing the %$#?%#? fender deck), you must realign the Quadrant. That's the metal part AM37574 or AM37575. Under the tractor there are 2 bolts holding this quadrant in place, they are between the frame rails under the fender deck right in front of the quadrant. Adjusting this part at the right place doesn't seem to be documented but you can do it logically. The shifter arm must move in the center of the quadrant's guides without touching it AND when the plastic "gate" (M48774) is down, the shifter arm must move just enough so that the fork's ball "clicks" in it's shaft groove.
Am I the only one to hate how JD made the fender deck so hard to install/remove on those tractors? %$#?#$?$@#.