Which gearbox selector?

Hi all!

I have to change the selector box of my PX, as the third gear "jump" very often. I read, in the past, that there are two kinds of selector box, depending on "something" on the 3rd gear position.

How can I understand which is the right selector I have to buy? Which is the parameter to understand which type of selector I actually have?

Thanks a lot for your help!

Max