Welcome, Dario.
I'm somewhat confused by what you are asking. From the way I'm reading it, it appears that you are trying to directly hard wire the machine to the prongs (A, B, C), foot control (H1, T)
and electric outlet. Usually, there is what is called a lead cord that plugs into the machine at A, B, C, with two wires coming from it. One goes to the electric outlet and the other to the foot control. I have not seen a plug (lead cord) shaped like yours.
Two examples can be found at
https://www.sewingpartsonline.com/le...g-machine.aspx
and
https://www.sewingpartsonline.com/le...al-ydk32a.aspx Neither of these will work with your machine. They are just examples of the type of cord one needs. One has electric connectors that would screw on to the foot control, the other has two bare wires that would go to the foot control.
It appears that there was a 503FA that was made in Italy and then a 503 that was made in Taiwan. It appears yours was the one made in Taiwan. It also appears that yours could be made to treadle.
Did you get a manual with it?
Janey - Neat people never make the exciting discoveries I do.