That's an easy fix;
One of four things is wrong;
1) either the M/S is moving fore and aft, or
2) the blocker rings are worn out or broken, or
3) the shift levers are overshifting,or
4) the strut spring is broken or has shifted out of it's anchored-position. This is the most likely.
If you are doing this on the bench with the cover off, I'm laughing. I know I shouldn't be be but it's hard not to. On the bench, you are the "over-shifter", Put the cover on and it will stop happening.
But before you do, those blocks, properly called struts, have to be properly located in the cut-outs provided for them, in the brass blocker-rings, AND the strut spring has to be anchored in the back of ONE of the struts, probably the one that is popping out, but the other end is the faulty side.. If you can lift the strut you might be able to see the small square-wire anchor pointing up at you. It has to be anchored into the hollow place in the back of one of the struts not necessarily the one that has popped out. You might have to go hunting for it. This HAS to be fixed or it will happen over and over.
Many times, the trans has to be disassembled to find the strut spring. And sometimes it is broken.
Oh I almost forgot, the spring of interest is on the other end of the strut that has popped out. Obviously this end must be there,lol or it wouldn't be popped up! But, you still need to prove the popped side spring is properly anchored in the back of a strut; if it is not, then it will eventually rotate out of position and you are back to square one;namely,dropping the trans.
If it continues to happen with the cover on, then the over-shift prevention mechanism is malfunctioning. This mech is in the cover between the shift levers. It consists of 2 balls, a sleeve and a short little pin that goes between the balls. Your pin is either missing, or too short. BTW, this pin is of super hard material, made to endure thousands of shifts. If you put a soft iron pin in there, it won't last very long. And it has to be of adequate fat-size so that it cannot get cocked in the sleeve. You can test the cover off the trans, just put it into any gear so that the detent caches it, then grab the other fork and attempt to shift it. It should not move more than a few thousands of an inch.
Nice pic; I had almost no clue what you were talking about until I saw the pic. Good job.
The three struts are indexed into that collar, also called a sleeve or a synchro-sleeve. They are held in place by the strut-springs. The struts are indexed to the blocker rings which prevent them from spinning.
When you start the shift, the fork slides the collar towards the selected gear. The collar drags the struts with it. The struts push the blocker rings up onto the conical brakes, attached to the gear being selected. This reduces the speed of the spinning selected gear, until it very closely matches the speed of the output shaft. At this time the collar is able to align all those pointy parts and complete the shift.
When the system is working right, this can all happen in micro-seconds.
When the system is not right, it just grinds everything into powder.
