Many reel-to-reel tape recorders visible in the film - set in 1976 - are Revox PR99s, easily identifiable by there digital counters, a model that was not introduced until 1982.
The type of license plate fitted on Dagmar's and the pimp's limousine was not in use in 1976. It should have been a two-line square plate.
The Schuko electric sockets seen in most interiors throughout the film was not widely in use except in kitchens in 1976. The sockets should have been two-pin unearthed round ones.
The brown milk cartons briefly seen in the borstal's kitchen were not in use in 1976, but came a few years later.