I'm afraid you're in possession of a (simple) CENTURY like sold in Germany and may be elsewhere.
This version has the (simple) XT-trigger (like TERRUS/PARRUS/CLASSUS) and thus a non-rotary piston without bearings and piston rod.
Have a look here.
What you're expecting is most probably the genuine CENTURY GT with the XM-trigger (like LGV/LGU) as generally sold to GB anyway also showing a non-rotary piston.
Much better gun due to the trigger system but both types with each a 30mm-system (stroke 88-89mm) are IMHO way to massive for sub-12fpe and in FAC-regions they are useless due to the long TP (about 24 x 3,3 mm).
I fettled ea one of this types and rate them quite useless.
Love my LGV .177 sub-12 and just buying another one.