I have two bikes with a CP gearbox, 1951 AJS 18S and 1951 Matchless G9. Both experienced shifting problems when the modern reproduction kick start bump stop rubber inside the gearbox disintegrated and the pieces started jamming things up.

The shifting spring-box made out of zinc in the outer cover are also known to crack (bad design) and cause problems like this.

The shifting mechanism on the later GB box is different.

I forgot a G3L CP gearbox is a grease lubricated box. So the kick start bump rubber won't have any oil to move it around. Unless it's a 1948 and later when they switched to oil.

Then I am still guessing it is the zinc spring box.

I'm thinking I'll just take it apart carefully and see where there is any wear or broken parts. Worst thing is that they are so messy being grease lubricated ! I'll post up what I find, it may take a while as we have other projects in front. Thank you for your thoughts Bruce.