A Lucky Cancellation To Be Able To See Ocp

I have found among my UK stamp 2 machin 6p, with the great fortune that the year 1972 appears in its cancellation, this means without a doubt that they are made of OCP paper, since there was no issue in FCP before that year. This has given me the opportunity to verify that under UV they have a shine that most of my other stamps do not have. So I have reviewed all my machin stamps of different values and I have been able to verify that special shine does not occur in stamps higher than 9p which are all FCP, with which I have been able to determine which are OCP and which FCP among the low values , OCPs even with UV light without a dark room shine noticeably brighter than the rest. I am almost sure that these cancellations have given me the key to be able to begin to distinguish at least between OCP AND FCP...

It is the other way around. FCP reflects longwave uv light brighter than OCP. They added an optical brightener. The clue is in the name 'fluorescent' coated paper.

In this photo, at the top there are different values, some of which are definitely FCO, at the bottom there are specimens that are definitely OCP...For me, the brightness is more intense on the bottom line...

Where do you think a coating goes?