SewOK,
That button holler fits the low straight shank Singers and any others that use that mounting system.
I've paid as little as $3.95 from a local thrift store to $15.00 or so from Good Will auctions. They are fairly common.
They usually come with only 4 cams so that one has a few extra. It should also have a large screw to attach the button holler to the machine and a feed dog cover plate with a smaller screw for it.
I'm looking for another one so if you are not going to use it and would like to sell it let me know.
Joe