this is what the hole for a case that takes the round end knee bar looks like. If you see a case where you see "Nothing" than either it takes the slot style bar, or the controller is missing.
Here are the two style ends - Singer changed to the Slot style around 1929 when they went to the bakelight plug in housing at the back of the machine to supply power. The round style bar is what came with the older style electrical with the male/female ends that plugged into the controller unit to supply power - and the machine was hardwired into the case.
BTW - this particular round end knee bar is a different shape because it's made for a very specific cabinet -and not to go in a case - otherwise the overall shape of the bars is identical and only the end is different.
I learned all this the hard way - spent $40 to get a knee bar only to find there were two styles of them, and I bought the wrong one!