Yeah - it looks like I guessed right - according to "How to Modify your Mini" by David Vizard, page 175: "The very latest thing in Mini wheels from the factory is, of course, the DeNovo 12 inch wheel. Basically this is a safety wheel and has the ability to remain perfectly drivable even in the event of a blowout. The 12-inch setup...."  ...OK - so 12" wheels were DeNovo's.  Now, flipping over to the Haynes Restoration Manual, we find that the 12" wheels came in for the 1275GT (only) in July 1974.  So I think it's pretty safe to say that we got it right here and the run flat tyre article needs to be fixed.  I'm off to do that right now.   Thanks for catching this one!