did I read the word "attachment"? you all are going to get pretty sick of me and my stuff!
Anyway - here are some button holers - the older style with the wing nuts, and the style with the cams.
The black and green cases have the same unit in them, the red typically has the slant for the 301. These all perform the zigzag pattern along with the size of the button hole.
This shows the extra set plus the eyelet - it's the one with the hole in it. Sellers often mistake the 3/16 cam for teh eyelet - the eyelet is rare and can sell for over $10 for just that one.
I want to point out the two screws in the front. The older style can come with either - one is identical to what is used with the hemstitcher. You don't need the funny shaped screw with the buttonholer, but you do with the hemstitcher.
So if you get a hemstitcher and need a screw, you can often get the old buttonholer and screw for LESS than people list just the screw for the hemstithcer. BTW - Singer gives the button holer screw the same part number with a -1 at the end. I can not see any difference, and they both screw into the same hole on the machine . . .
Here is the next group - a bit newer and often with plastic cams
These come in slant and straight, and still zigzag.
And how do you tell them apart - it's rather obvious, but you'd be surprised how many sellers don't know this.
And the last - these are for zigzag machines. They are the same except the feed dog cover plate, though both will work on the 401. Keep in mind, if you have a 404, you need the style that will zigzag.
There are many other brands out there than Singer, many made by Greist (I believe the Singers are), though some are for top clamping machines and are made to connect to round bar instead of the flat sided one that Singer has.
I don't have a top clamp buttonholer yet (it's in the mail LOL!!)