I do not know the english word, in german it is "Lüsterklemme". It is a connector you see in the following picture. Important is, that it is of ceramic or porcelain. If you see the part, you at once know, what I mean.
In my sys 1 machines I screw this part to the cabinet and then connect the NTC. The lead of the NTC I put through BOTH srews - but pictures say more than words, so please see the picture.
Always be aware, that the old Gottiebs are imho dangerous inside the cabinet concerning safety. There is no good isolation of nearly all HOT mains wires. I never would allow my my children and my guest, to touch anything inside the cabinet. From the elctric point of view from today, it is criminal dangerous, only my few cents.
Hope, the picture helps you.
Varistor is easier to install, you can simpy solder it the the input of the line filter.
NTC Ceramic Connector Sys1 trafoboard (resized).png