I can buy a replacement gear for the auger ($75) on line for any year for the 521 EXCEPT the year of my machine which is 1989. The part number for the gear part on my 1989 machine that is no longer available is 33-9050. Up to 1988 it was part 23-9050 and from 1990 forward part 23-9050 was replaced by part 5-7180. Why is this gear different for only one year, 1989, and why does part 23-9050 and the part 5-7180 not work for the 1989 machine? What did they do differently that one year, 1989, that makes this part unique and non-replaceable?