Possibly M39 x 3.0mm?
Close but it is not 3.0. It really is 8tpi. This was confirmed with a thread pitch gauge.
Major diameter is 40.1 and that would mean M40 and make life easy, but the thread really is nothing metric and even should M40x3 fit it will not be snug.
1"1/2 is significantly smaller (38.1mm)
1"5/8 is significantly bigger (41.2mm)
40.1 mm is close to 1"9/16
A Philips Constant service guy answered my mail last week: they have nothing anymore about this machine (or he doesn't want to search for it)
I found another imperial thread. It is a mix as is the case with many machines of that era.