It's late now, so I'll have to look back through my research. I think Disraeli Gears talks about it. Mine matches the image of the third version. They "fixed" what was wrong with it. I can take a photo tomorrow or so.

First came the 4900 (1982), then the longer cage & redesigned 5500 (1982), then the Mountech II (1985), which is this one, and the one I have.

http://www.disraeligears.co.uk/Site/...erailleur.html