I have! And have one on my M - I bought it a number of years back on a 4 speed box. I was missing the rearmost aluminium casting, and had to fabricate something to serve the same purpose.
Thanks for starting this thread Richard. I have been wondering who made these and what they looked like when new.
This is the one I have -
The aluminium casting close to the gear lever seems to have been the Achilles' heel in what would otherwise be an effective design. The welded on steel bracket on mine is clearly an old substitute for a disintegrated original.
Apparently these will fit both three and four speed gearboxes and can work very well.
I have been trying to find an advertisement for one of these but without success. Apparently they work well and so far as I can tell will fit a three speed as well as a four speed gearbox. So far there is no evidence that this is a Derrington product.
Derrington seem to have made a better job of advertising their three speed remote but I have yet to see an ad for Derrington's very rare four speed remote .