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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