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Thank to the community: I´m sure that this bike is a "PMX"!
After the community gave me some interesting hints, now I´m sure of the origin of that bike: it´s an (1982? - see s/n R38543) PMX Club.
It was sold to me as an oldschool GT, and there were only the following parts: frame, fork, stem, handlebar (alloy), one-piece-crank, seatpost - the rest is NOS or/and used old stuff from myself, e.g. the wheels: 1982 S.I.C. BMX (as stamped on the rims).