Well, in the last couple of weeks MMC must have gone back to their old ways!

They were great in the old days. But the "old days" is a relative term. They used to be the fastest least inexpensive source for parts in the world, commanding a lot of loyalty in the community. But times change. Production changes became daily and at one point, parts sourcing and supply split into two departments, one for Production and the other for the masses of cars on the road, called Parts, curiously enough. It didn't work out well for the latter group.
It may be that a different cable fitting was used for the R380 and there was only a small number of LT77 compatible cables around.
Though very different,
aside from gear ratios, the 2WD R380 was made to be compatible with the 2WD LT77. So the gearbox end of the speedo cable is identical to the earlier LT77 cable. The same internal gears that drive the cable at different speeds, depending on the car in question, are interchangeable. In this case, the culprit for the mistake, is sourcing again. The Parts department was told it was a seamless swap. The driving force behind sourcing has changed since the regime change 2+decades ago.
The issue here is not the R380 side. It is the change at the other end, speedometer.
Sometime in 1994 Land Rover changed the Rover v8 gearbox from the LT77 to the R380 which presumably was used until 2004 when the +8 was discontinued. It may be that a different cable fitting was used for the R380 and there was only a small number of LT77 compatible cables around.
Well done! Minor codicil. The earlier R380s (up to early 1999 from memory) have a minor that can be cured with the standard rebuild kits sold after. That being said, I have never run across a Morgan R380 that needed it.
L.