OK, here goes - the earlier LDs, up to around 1956 or 1957, had two 16004 bearings supporting the pinion, each bearing being 8mm wide. The later Italian ones had a 6004 bearing (12mm wide) closest to the pinion teeth, and a 16004 bearing at the other side of the spacer, which was 4mm thinner. The reason for the change was that the bearing closest to the pinion teeth took most of the load, and the 16004 bearing just wasn't up to the job, especially after a few thousand miles. (I had one of these collapse without warning on a Mk2 LD - it removed all the teeth from the pinion, and a couple from the crownwheel.)
The French LD125 Mk4 used a 6004 bearing at each end, with a spacer which was a further 4mm thinner, and the same type of oilseal (10mm thick) between the two bearings. I think the only reason for doing that was for production reasons - 6004 bearings are generally cheaper than 16004 bearings, and there's no chance of getting the thinner bearing in the wrong place during assembly.
I'm currently assembling a tailcase with a French 125 crownwheel & pinion, and I'm using the 2 x 6004 setup. I'm also using an oilseal which is only 8mm thick, mainly because it's less than half the price of a 10mm thick one!
