Extremely rare ZX Microdrive Cartridge on Ebay

[RWAP]

Has anyone ever seen one of these before:
http://rover.ebay.com/rover/1/710-53481 ... =824&kw=lg

(I admit it, it is my auction). I guess it was produced for promotion material to show the inner workings of the microdrive, although from memory, I don't remember a picture of one - just hand drawn diagrams....

Interesting!