Probably a collector's "mark". Back in the day many collectors had small "owners stamps" made up to stamp the back of stamps added to their collections. Similar to and serving the same purpose as bookplates that people put on the frontispiece of books in their library.
Given the overprint, it is more likely an expertiser's mark. There are sites on the internet that list ("catalogue") expertiser's marks and identify some of the most important ones through history.
There was a previous long discussion of expertisers' marks earlier this month on this discussion board. Maybe somebody can find it an add a link -- I am being called away from my desk right now.
Roy
https://www.filatelia.fi/experts/index.html
Don
SOME TIME AGO I GOT A LOT OFF E-BAY DESCRIBED AS SIGNED PI C29-C35 Von Gronau AIR MAIL issues this set is as the same as the normal set but on the reverse in the lower left corner there is stamped in red ARH ?? not in my specialized cat. Does anyone have any insight ??
re: Philippine VON gronau C-31
Probably a collector's "mark". Back in the day many collectors had small "owners stamps" made up to stamp the back of stamps added to their collections. Similar to and serving the same purpose as bookplates that people put on the frontispiece of books in their library.
re: Philippine VON gronau C-31
Given the overprint, it is more likely an expertiser's mark. There are sites on the internet that list ("catalogue") expertiser's marks and identify some of the most important ones through history.
There was a previous long discussion of expertisers' marks earlier this month on this discussion board. Maybe somebody can find it an add a link -- I am being called away from my desk right now.
Roy
re: Philippine VON gronau C-31
https://www.filatelia.fi/experts/index.html
Don