Nikkormat FTn, or something like that?!

[BernardL]

Is it a sound idea to use some drops of naphta?

I would not do that. Also below the lens flange, I suspect there is a wiper resistor to transform the aperture information into an electrical resistance for the meter circuit. You risk to transfer "goo" onto the resistive track. More generally, you don't know where the naphta carries the dirt.
I would remove the flange look-see what's inside, and clean selectively. Some contact cleaner on the resistive track and the wiper would be in order.

 

[Sumarongi]

I would not do that. Also below the lens flange, I suspect there is a wiper resistor to transform the aperture information into an electrical resistance for the meter circuit. You risk to transfer "goo" onto the resistive track. More generally, you don't know where the naphta carries the dirt.
I would remove the flange look-see what's inside, and clean selectively. Some contact cleaner on the resistive track and the wiper would be in order.

Thank you, BernardL. —— "contact cleaner", what is that exactly? Does a hardware store (ironmongery) sell that stuff?

 

[Sumarongi]

My second FTn,

[...] (made in 1974 I suppose, serial # FTN4753***) [...]

is this here:

Dear Nikon experts,

IMHO, this doesn't look at all like a Nikon FTn made in 1974, does it?

I'm quite certain that serial #s like FTN4753*** were made in 1974, but why does this specimen have all the characteristica of a 1967/68 manufactured FTn?

BTW, an additional mystery: the serial # FTN3547*** does only have a «modern» cable release thread, while the FTN4753*** has both the female thread *and* the male thread for ancient Leica cable releases...