A Tale of Two Gearboxes

[ramwing7]

I have a 1949 BSA B31.  The gearbox is a mess so I got another hoping to use two to make one serviceable.
I've come up with a situation that I cannot find any information on.
In one gearbox the mainshaft gear (24-4231) slides on the shaft easily.  In the other it took 5 tons of pressure to remove the gear from the shaft and if it goes back on, it will take a press to get it there. The easily moveable one will not slide on the other shaft either.
Question is:  Which it correct?
                   Does the mainshaft gear move along the mainshaft in normal operation or is it supposed to be fixed and stationary during operation?

Many Thanks.

[Rex]

Assemble the boxes and see which one works, or look at the assembly drawing/gear positions and see?

[R]

Aye.
Since one is not like the other, something is badly wrong here !
Can you see if the free one is bushed inside (as it should be ?)

If that gear was fixed to the shaft, then any notion of 'constant mesh' gears goes out the window !
And since the fixing seems none too permanent, press required regardless, seems wrong....

https://i.ebayimg.com/images/g/nV8AAOSwv~NlsRlm/s-l1600.jpg