Sir F. A. Rien wrote :

It may help. The lightsource should be a very low angle to emphasize
the 'height' between cancel and OP.

As Most GB OP's are shiny black ink, if there's any 'shine' above the
cancel, it's suspect and neds further checking - usually witha 3D
scope or genuine OP. Differences between genuine and fake are slight
but are there for study.

I don't really bother with that as even the Royal has been fooled.
Strictly the 3D [stereo] microscope.

I've thought of trying to make an anaglyph usingthe macro feature of
a good digital camera. Camera on stand, shift the stamp 2 mm [more?
less?] for the second exposure. Haven't clue if that would show
enough!

Thank you for this additional information.
Nise idea, the anaglyph !

--
All the best,
Pierre