I’ve just gone through this minefield when rewiring a 1960s Singer machine (model 120k if memory serves). There’s an awful lot of confusing and contradictory stuff on the interwebs (said he, trying not to sound surprised) so I went back to first principles.
For that particular model I concluded that the construction of the electrical components, which form a self-contained unit, could safely be treated as class 2 so I wired and tested it accordingly. I have upwards of 50 years electrical engineering experience, and am PAT certified.
There will now be a short intermission while the barrack-room lawyers marshal their arguments. I’ll let you know how things went after I’m released from chokey (if they can find me a room to share with all these rioters).
George