This error came from the Harlow Post Office Post & Go machine, which had the Birds II stamps in it for ONLY FOUR DAYS in February 2011!!
Harlow Post Office was a non Birds office at the time and should have only issued the standard Machin type faststamps!
However nearby Milton Keynes Post Office ran out of Post & Go Faststamps at the end of 2010 and Harlow PO let them borrow some of theirs.
When Milton Keynes eventually got new stock in it was all Birds II stamps and to repay Harlow, they sent them Birds II instead of Machin type faststamps, which they put in their machine!!!
When Royal Mail discovered the mistake they instructed the staff at the PO to withdraw the Birds II stamps immediately and replace them with Machin type faststamps which they did on 23/02/11.
All machines have now been upgraded to the smaller font and Birds series 3 stamps.
UPDATE:- This unique set is available here
Whilst looking through some ‘Hytech’ vended Post and Go Stamps I have come across some where the security overprint does not cover the whole of the stamp~there is a discernible and noticable gap at the base of the stamp where the print does not reach the bottom of the stamp. Upon checking with other dealers they have noticed the same on a handful of stamps purchased at Spring Stampex 2011 and have confirmed that at least one whole strip of five with this error on each stamp exists.
A contributor to this site has reported a nice ‘Miscut’ variety from the Finchley, LondonÂ Post&Go Machine, along with the receipt that is generated when the machine detects such an error. Nice One!
The receipt reads “We apologise some items could not be printed, Please present this part of receipt to a member of staff for assistance”.