Hi gizzy,
33d6 has answered your questions,
The boxes with reverse gear fitted are easily spotted because they have a "bump" on the main casing where the reversing gear fits
I got a book on them on ebay some years ago,
Like 33d6 I have a couple of "not quite right" boxes
( bigger than the one you have)
But, I managed to find enough bits to build a 4 speed to go into my wee OK Supreme
I'm sure the extra gear will help a lot
All I need is time to get round to doing the change over
John