Right, it is a 'standard' Three-Pin Terminal Plug, or at least that is what they call it in the adjuster's manual. I did look it up.at sew-classic and one page it was called 3-prong, but on another 3-pin. Several Singer machines have that type of plug and terminal.
So the 99k has two plugs (one from the motor and one from the foot controller) that go into what looks similar to an electric outlet near the hand hand wheel or inside the cabinet? That is called a motor block.
That is super that the 99k came in a cabinet and at a good price. Since it is what is considered a 3/4 machine there weren't many cabinets for them, like for the full size machines. I think I have only seen one in real life and they wanted more than I was willing to pay.
Janey - Neat people never make the exciting discoveries I do.