Thanks Risto, this is really helpful.
the watch I’ve seen is being sold by a London watch dealer and that is the only photo I have. Incidentally, it has had its original 861 movement replaced with the same movement. Apparently this was carried out in the 80’s. It does mean they have been unable to obtain an Extract of the Archives.
it’s a lovely watch but I’m minded to steer clear of purchasing it, mainly for that reason plus my main focus is on something else.