[singleminded]

 when they changed to an AU numbering sequence and needed a different primary chain case.

now there's another story in itself.

[VicYouel]

Having had six 4 speed gearboxes for the thirties the were numbered AW or BW with four digits until 1934/5 when they changed to an AU numbering sequence and needed a different primary chain case.
IN 1932  they had a solid shaft with a spiral lift off mechanism in the chaincase cover and then reverted to the conventional mechanism on the off side. The drive  side cover had variations and internally the layshaft had solid or an assembly of bits. 
Vic

[wessex_man]

As Paul says it's a case of piecing together the various spare part books, and the box is not the complete story you have to look at the shafts, gearing and the Clutch. I guess if you have all that as one assembly and the chains line up you could transplant one set from one year to another to keep your bike working. I've never really looked at the detail in doing this though.

I've found some pictures of a later AU box as fitted to my 1935 9A which I think found a new home in NZ.
This had one of those bearings in the clutch center as was very fiddley to put together.

[phutton]

The gearbox used in 1932 and 1933 was the AW-prefix unit, which was replaced in 1934 by the AU type. The rack-operated four speed constant-mesh box from which both were developed was introduced in 1924 only on the Model 7, and was itself developed from the 3-speed rack box first used in 1915.

As you say, the AW had a solid mainshaft and the pull-off clutch mechanism, while the AU reverted to a hollow mainshaft with the usual bridge-type clutch mech. As usual, however, that is not the whole story, and you will need to analyze all the parts lists to find the detail differences.

P.

[wessex_man]

I'll see what pics I have.

added 28/12/2016  Here's a starting point from the 1932 spare parts book

[singleminded]

Rather than have this floating in the Xmas message thread lets see if the various 4 speed boxes can be identified here.
I know of the 20's type with the straight pull rod that operates from the top rear of the box(think I'm right). I'm told these could be fitted into the early 30's as well.
My box looks similar to a 3 speed but the selector lever comes out of the righthand side at the bottom of the box . Undrilled mainshaft as it is the 'odd' 1932 system that pulls the clutch from the primary case.I'll post a pic at some time as I have it parked at my inlaws whilst i do his 1931 9A.