Thanks Halfpint. From what I understand, the metals ones were fitted to the Morris 1100S and the 1100 automatics (and perhaps the Minimatic) as well as the early Mokes. A local bloke over this way that I've been getting a few bits off has one on an 1100S but he wants to keep the running gear all complete and sell it as a complete unit. Evidently, the 1100S has a 1275 engine that is sort after for modifying. Another bloke who was into Mokes reckons the early models had them.
Somerford Mini (UK) and Minispares show the metal ones on their part lists but they are no longer available. The part number for the metal ones is 12G4298 and the plastic is 12G4299.
The metal ones look a lot better than the plastic ones, especially for an older model. Evidently, the metal ones also don't warp as the plastic ones may tend to.
I'll keep hunting - hopefully someone will have one in their shed.
Cheers, Rocky