if you can unlock it by jigging the lever outside the box, you need to fine-tune the adjustments - never any need to buy a different box, just get yours adjusted properly, the forks and the pawl that moves them must be exactly right, after that's set, then you adjust the trans rods at the outer levers with everything in neutral
the only thing that can go 'bad' is one of the forks getting the corners worn so rounded off it lets the pawl get between the 2 forks, and that can be fixed with a bit up weld filed back to get things "new" again - the wear along with the getting stuck is the result of sloppy movement thru the H pattern, always move the shift lever along the H lines, don't "cut corners" ....
here's a 55-9 box, same as yours inside, note the space between the forks is less than the thickness of the pawl, and the inside corners are 'crisp and square'
Bill
