Product review (1 stars): Wrong part number?
The Sinkmaster Disposer has a very prominent model number and other information.  The Lasco 3909069 is the part, according to them, that I needed to replace.  The stopper was inexpensive but, by the time shipping was included, this was pretty steeply priced for a stopper.  Even more expensive when it turned out to be about 1/8 inch shy of the diameter required.  Double-checked everything -- all was correct -- and then chucked the stopper in the recycling bin and bought one of those flat rubber things that replies on water pressure to hold it in place.