Gloucesternige wrote:

...The pic that Dave kindly posted is looking forward inside the main box housing, ie:-, that shaft at the bottom of the pic is the rear end of the input shaft. The two holes to the right of the “thing” are where the two forward detents press in. ...

Thanks, Nige. That's what I had come round to after further thought. So the main selector shaft is absent from the photo, but if it was fitted the front end would protrude into the tube under the breather (on an assembled box you can see it down the breather hole. I'm beginning to understand!

I'm fighting the urge to take one apart! Must look out for a cheap one.

ashtrans wrote:

the front 2 detents have to come out, rear 2 can be punched through later once the rails are out,

we drill the front 2 and use a slide hammer, this scraps them,

blackwolf wrote:

If you remove the breather with the gearbox otherwise complete, is there anything to stop the semi-circular thing that the breather screws into from rotating round the selector shaft so that the thread is no longer at the top? It obviously can't fall off, but if it inverted itself I imagine it could be very difficult to get it back in place without stripping the box.