There shouldn't be any external current in the machine body, even by 1960 standard. By 1950 the double insulation mark was pretty much standard on most electrical appliances, at least in Nothern Europe. My 1950 201 has it. I think Singer kept the same high standard were every the machines were made, even though different countries had different standards. Very few domestic machines had three core grounded wire (the same for modern machines). I have only seen a single 201K23 with three core grounding so far. If there is any problem, have the wiring checked, a clever guy will find soultion.