The origin of this overprint is unknown.
This overprint is currently unlisted by Stanley Gibbons but a reference was made to it in an article in the 1926 British Philatelist.
I only know of three examples of this overprint - two I own and one reportedly in another private collection.
Unofficial overprint type ?? (unlisted) in dark blue.
PP? (unlisted) 1d rose-red (SG40 or C10)