Norbert Michailowitsch, one of the regulars on the classicopels board, provided this information:
Due to the intensified exhaust regulations starting from chassis
number 2265862 changed engine parts were used.
Opel changed the cam shaft, the cylinder head with hydraulic valve
adjustment, the valves and the (low compression) pistons.
The result of these modifications is a power loss, to a lower
compression of 7,6:1 (78 HP - 4800/min).
The engines were characterized as 19SUS and could be operated with 91
Starting from the chassis number 2560567 the vehicles were equipped
with the O.E.C.S (Opel Emission Control System).
Starting from the chassis number OY07NC2944586 the power of the
engines is indicated as 75 HP at 4800/min.
In Germany we remained exempted from these restrictions of
Opel GT World