
Well known too collectors of this epoch.The London forgeries have fascinated many.For reason;simply because on cover they are very rare.Among the ones authentified are
addressed to Cecile Lefort who was a British agent(SOE)sent too France.
In fact very little is known.It is assumed that agent's destroyed the letters immeadiatly
as a security measure.The Lisbonne address was a transit address used by the Intelligence Service
that forwarded the contents too the agent.The stamps had secret marks that identified that the
sender was an agent (French resistance) thus a guarantee that the agent was an authentic agent.
She was in (SOE) Section F
Grade;Section Officer
Field name:"Alice"
Birth name:Cecile Margot Mackenzie (married: Lefort)
Died:Ravensbruck (Deported)
In the links below the information on the various types and an article in english.

https://www.mysticstamp.com/info/fr...-spy-stamps/http://actl.forumgratuit.org/t80-faux-de-l-ishttp://museedelaresistanceenligne.o...-britanniquehttp://www.timbresmag.com/wp-conten...eLondres.pdf