[MENTION=413981]markwesti[/MENTION]

as Andy noted, there is quite a bit of (especially small part) part interchangeability between Shimano parts. I can't be 100% certain without you posting a picture, but it sounds like you are describing a threaded post in the derailleur body, secured by two flats. If so, rejoice; this is still a common design on low-end Shimano parts. You may be able to find an STX derailleur for cheap, for instance, that may have a piece that will fit. A bike cooperative, with a large parts graveyard, would be invaluable here. The fact that it is secured by an external hex nut notwithstanding, it should work fine, with the use of a nut driver.

By the way, is this the model you are referring to?