That looks like the slot that works with a bottom clamping machine, like a White Rotary, or early Kenmore Rotary, or Standard Rotary as examples. I have been looking for attachments for my White Family Rotary. I only have a straight stitch foot with it now so I am interested in any hemmed or braiding foot/plate. The thing is, all the bottom clamping feet are not interchangeable (as I learned when I tried to use the attachments from my Standard Rotary on my White Family Rotary ). The measurement of the offset (from the top of the attachment slot to the bottom of the foot) is 1/2 inch or slightly less, like maybe 7/16 inch by my measure. The width of the slot is 3/8 inch and the distance from the needle hole to the front of the slot (looking from the top) is 5/8 inch.
if those measurements match your attachments, please PM me with the particulars.
There was a chart I saw somewhere, Needlebar maybe?, that gave the measurements for the feet of various bottom clamping brands. Maybe someone else knows where it is.