France defs - 1955 allegory of "France"

I have several copies of the 15f red, 20f blue, and 25f red of this
issue.

As part of an ongoing investigation into all my world "common"
definitives I have eaxamined these under a lw u/v lamp.

To my surpsise I find all 3 on a paper with no reaction and also all 3
with various intensities of reaction from a quite dull to a quite but
not intense bright - the reactions being on copies mostly postmarked
between 1957 and 1959 ( where a date can be identified ).

There is no mention of this variation in SG specialised France so I
surmise that any change in paper-coating is not in connection with
postal mechanisation, but purely a formulation which flouresces "by
accident".

Does anyone have any information, theories or just plain guesses as to
what happened here?

I should add there are quite different shades - or possibly intensity
of colour also, but this seems to be unconnected with the degree of
flourescence although frankly I do not have enough copies of any of
them to come to any conclusion , with a minimum of 4 and maximum of 8
copies.

It seems to me that anyome with a large number of copies of this issue
has the op****tunity of some meaningful reasearch here.

Any cerebral input gratefully received.

Malcolm