This is indeed a genuine CCS 25 built on a differently machined and heat treated B 25 20 ga action

Hi, that's an interesting issue, it is definitely a FN 25 but from the photos you can not state that it was built on a 20 ga action. In fact some FN 25 doubles were/are built on B25 20 ga actions but others are made specifically as double rifles and these are the true CCS25. There are some minor but distinctive differences.
As some of us are interested in these obscure themes, I will be back with some photos to point out the differences.
Meanwhile here go some pics of my CCS (which is a CCS25 built from the begining to be double)

Pictures 2 and 3 are important to determin if the action was built for a shotgun or a double. Picture 4 helps to set 25 models apart from 125 ones.
I'll be back soon with the explanation.