Well in the first place, a 1957 did not come with an ignition switch. Only a start button and a choke button. So, you need to figure out if it was modified for an ignition switch. Coming from under the magneto will be the two spark plug wires. Also, as it came from the factory, there will be a third, smaller wire going to the vacuum switch. Now, the question is, do you see a fourth (small) wire?