The ABS Sewer Pipe on my RV is standard 3" ABS, which has a measurement of 3" ID, and 3-1/2" OD (1/4" wall thickness).
The dimensions of the Caps that I have for the Sewer Drain are all 3-1/2" when measured between the locking tabs.
The Valterra and CAMCO RV Sewer Caps sold here in the US all have the 3-1/2" measurement, with the exception of the old style Thetford items mentioned in post #2.
I tried, but could not find, the dimension of the CAMEC product that is available to you there. That is something that you will have to do by contacting a supplier and have them measure. The only information that I could find on the CAMEC item was the listing in their catelog, and that gave no dimensions.
Hope this helps.