that style base with the knee bar with the barrel end was produced in the 20's, up to about 1928. At that point Singer changed the wiring and went to the plug in connection at the back of the machine instead of using the chicago plugs.
This is the later style wiring for the knee bar cased machines - uses a bar with a flat end.
also, see that raised plate right below the badge? that is the serial number, there should be letter(s) in front. Make sure you've cleaned it off so you can see all the numbers.