Bonnie, you are right about it not being for a FW, but
it is a Singer card table for a different machine.
I think there are a couple of ones for the 301. Here is the link for what they call the 314 card table -
https://ismacs.net/singer_sewing_mac...umber-314.html which they say is for a 301 long bed. The dimensions they give for the cut out is 19 1/4" x 7". They don't indicate another one, but mine is 16 1/2" x 7" and shaped insert looks the same. I was able to almost get a 401 in mine which doesn't have the notch on the end and just rectangular. It didn't fit quite flush but I was able to sew with it.
Here is the link for the 301 card table for the 400-500 series and says for short bed 301
https://ismacs.net/singer_sewing_mac...0-500-301.html. Note a cut out for the plug on this one.
The featherweight insert goes all the way to the edge as seen for table 308 at
https://ismacs.net/singer_sewing_mac...308_table.html