Clive,
After replying last night I checked a few parts books of the 50's era, and would think that driveshafts other than those listed under the vehicle makers code, ie LV9/BOE, LV7/BD, etc., several came under LV7. As LV7 list not only included Eaton transmissions, engine manufacturers, etc, I conclude that they initially thought it should show driveshafts and components. As an example, The Commer Q4 4x4 parts list show two types of drive shaft fitted, one is the old Layrub coupling type, with parts prefix of LV7/LEG (Laycock Engineering), and the normal u/j type as LV7/HP (Hardy Spicer). I think the change to 6MT10 might have come in early 60's.
The larger shaft you have with a NSN might well have been a modification introduced when the heavier Mk2 Pig was in use in NI, and unless you have an updated parts list, it will not be shown. Or it could be that stocks had run out of the original type and new shafts procured of a larger tube size, thus making it another part number as design had changed.