I'm a bit confused.
Correct - Caithness is marking their seconds "CIIG" - but if this were a Caithness second, no design number would be scratched on the base. If a code starting with a single(!) letter, followed by numbers is marked on the base, this would be a running number for unlimited weight for a given year (indicated by the letter) - but NEVER with seconds.
I am not saying this is not Caithness - but finding the design will be difficult - with more than 4000 designs around.