Sir F. A. Rien wrote:

I've not seen them on any GB issues I have.
Judging by the UL stamp, they appear to be placed on the stamps after
cancellation! Only a physical examination could determine if this is
correct, but the red seems to cover the CDS.

"Out of the Blue" - a child playing Postmaster with a rubber stamp?

Thanks, Sir F
Yes, maybe it's a child. We have seen that before!
--
Mvh Toke

http://www.norbyhus.dk/