Quote:I don't know about this Guru thing, I feel more like a cynic.
OK, we now know that it is 'different', but is it 'interchangeable'? Does the old number supercede to the new number? I suspect that it might. [I hope so]

So from the bulletin that you quote, i read that
A. The gears are made wider, & therefore a little stronger, & that can only be a good thing
B. The main shaft bearing is strengthened. [But which one, pilot, or main support bearing in the sandwich plate, & how is it strengthened?]
C. The bearing at the front of the countershaft is strengthened.
D. there was a change in the extention housing to help get oil to the rear bush.

I'm guessing here, but this is what I think.
Point 'A' needs no explanation.
Point 'B & C'. I wonder if they simply case hardened the countershaft front snout as it now becomes part of the bearing itself, & must endure not only the side load from the constant mesh gears when in the intermediate ratios, but now it must also serve as a wearing element of the bearing itself.
As for the mainshaft, I suspect that the pilot bearing hardness was either increased, or made deeper, or both, as this is also a high load area when in intermediate ratios.
I have never heard of early 56 series gearboxes having a bearing problem at the front of the cluster or at the mainshaft support [ball] bearing in the sandwich plate, but the mainshaft pilot bearing is a common problem area in many gearbox designs.

I suspect that there might be a bit of 'spin' used here when describing these changes & if the 56A countershaft is interchangeable with the earlier 56 item, then I suggest that an even stronger box could be built by using a 1000 or 1200's 56 gearbox case & front bearing on a 56A gearbox. [The ultimate 56 series box?]
I notice that there is no mention of the obvious cost saving benefits to the change to the countershaft bearing.
Well, that's what I reckon anyway.

Have a look at the bearing on the countershaft in the photo I posted earlier. That just has to be stronger than the much smaller pressed tin race that they use in the later box.