Campy C Record cranks came equipped with self extracting crank bolts ( I believe they were the first crank to do so) . The "dust cap" in question is possibly the extractor head, in which case you need to leave it in the crank.

The crank bolt itself is the standard 8mm ISO metric fine RH thread but the extractor head is 22mm LH thread. A standard crank remover will naturally be threaded the other way.

By the way the extractor head is in a plane normal to the axis of the BB spindle while the surface of the crank itself slopes out to the pedal (to improve Q factor). If you are not used to this it may seem like the extractor head is not straight but it's easy to check - rotate the crank and check the runout on the extractor head.