I am guessing that once upon a time, there was a "clip" (for lack of a better word) that you twisted to hold the machine in place. I'm "picturing" the clips that held a sewing machine in a "box". Sorry, I'm not coming up with the right words.
In the olden days, a sewing machine that wasn't sold in a cabinet was sold in a wooden box with some sort of a top. They had little "clips" that held the machine to the bottom of the box. So that you could oil the machine or do some other chore, the "clips" twisted (pivoted) to allow you to tilt the machine back.
I'm sure they have a name, but I don't know what it is.
bkay