I've acquired a set of original (I believe, has the hella logo) euro lens, when installing them I couldn't get the one finger nut on, bottom outside. I took it back off and found the stud wasn't threaded.
Anyone seen this before?

It seems to fit OK without it, but it messes with my OCD. I'd could just thread it but it looks thick, like there should also be a step in it.

Should I try to thread it?
Or just say fuch it and don't risk messing with it?

I would leave it. 50 year old plastic gets brittle. If you snap the stud off or even just crack it, you will be bummed. Enjoy it for its oddity.

I'd probably leave it as mepstein says brittle plastic. Maybe you could get a spare knob and drill out the thread so it fits over it to disguise it and look like the others.

I would try to tap it since I too have OCD issues. Heating up the plastic might help w/r to the brittleness.

I'd probaby try to thread it, vice locks around the stem to not have the torque go into the plastic.....

I was thinking the same thing.

I agree with above, drill out the plastic finger nut just enough so you have a tight friction fit.

KISS w/ no shortcuts.

OCD

- reminds of an old story by my major prof about when he was digging cesspools as a summer job & wanted to cut corners at end of day, since once the pit was filled with SH***T nobody would know if the bricks had been perfectly mortared ....

- his boss said "i will know, Dick; I will know"

or - is it we presume some PCA Parade judge will notice the lack of threads?

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+1 on OCD. There's no way I could let that go uncorrected.