"S. S." wrote:

As the subject suggested this is a stupid question from a newbie. I
wonder what is the sticky glue-like stuff in dark color that is near
the counterweight whole where the armtube goes through.

This is only a guess but frequently there is some kind of
calibration guage or markings on the front of the counterweight.
On some models this was accomplished using a printed label with
an adhesive backing that was stuck to the front of the
counterweight assembly. If your calibration guage is missing, the
sticky residue may be what was left of its adhesive backing where
it was attached to the counterweight.

- Jeff