Richard, it's odd that that is the only large prop shaft I've seen fitted. Unless of course our readers know differently!
I have had the privilege of looking quite a lot of pigs, but not nearly enough! Clearly only a small fraction of the total.
Although there are EMERs that introduce new components they are, as you well know, only EMERs because something has to be modified to accommodate the change.
So there must have been, as you suggest, a substitution. Although in the IPLs I've seen, I have never seen an amendment for the prop shaft. But I expect it was just a single sheet of paper lost in the mists of time.
But it would be nice to pin down what this prop shaft was also used in, so that in future years when we're dead & gone, the new owners will find my list of Humber equivalents to keep them running. Sobering really that our vehicles will last longer than most of us!