I am sure back in the day of making Wallis tractors nothing got thrown away. If they had an engine go bad for some reason and it was replaced by warranty the old engine got sent back to the factory and refurbished and then in turn got sent out to the next person needing an engine. This would explain the older engine verses the newer engine.
Wallis tractors were forever changing during production from what I can tell by all the tractors I have personally come across. I think if they found a weak spot they changed it the next day or next pouring of cast to beef it up or make things more accessible.
Of the three Cub jrs I have here all three have different transmition housings, one with the long shaft sleeve, one with short shaft sleeve and the other with a short shaft sleeve and access cover for the clutch pivot. You can see the difference once you see them all together.
Graham, yours should have the later one with the access cover am I right??