Ah, thanks Gorgonops!  That is some really good information to know and cleared up some confusion on why some drives (IIgs styled drive A9M0106) work.  I was so curious about this that I had actually purchased an M0131 drive off of eBay because the whole scenario seemed weird.  It hasn't arrived yet, but now i know.  It would be interesting to find the time frame (serial number-wise) the drives were "fixed".  I'm also curious if the mechanisms are interchangeable between the housings.