Now that's interesting, all the '4-holes' I've seen before have a narrow band clamp like this: http://www.classiclightweights.co.uk/co ... w-band.JPG
..Does that make it particularly rare?
(I have one the same, and yours is the only other I've seen)
Yes, the ones I have also have the narrow band. I also know the narrow bands nearly all split around the pivot point so I can only imagine this one had had it's band swapped with the older/newer/other type. The bike i got it from also had a home-made back plate for the NR r/mech so I guess the previous owner was a tinkerer.