Sir F. A. Rien wrote :


Another trick is to use a 20x loop and a light source coming across,
not down, on the stamp. With anything but a good cancel, you can pick
up the OP ink resting -=on- the cancel.

For GB material, only 1 in 10 is genuine !



Ah ah !!
Quite interesting :-)

May be, I have to search within my GB stamps for those with overprints
and faint cancels.

Then look at them with my USB microscope and a light source coming from
the side (if I understood correctly your explanation).

--
Kind regards,
Pierre Courtiade