The British Postal Museum & Archive will sell duplicate imprimaturs and registration sheets at two Sotheby’s auctions in July 2013 and February 2014.

Two registration sheets were printed for each issue, and the BPMA will always retain one of them. The chance to buy the other is therefore a once-in-a-lifetime opportunity for Great Britain collectors.

The items in the July 11 sale will include registration blocks of the King George V ‘Seahorses’, King Edward VIII and King George VI definitives, as well as imprimaturs taken from the sheets with archival notes in the margin.

The 191 lots are estimated to raise up to £5m, with the proceeds helping to fund the BPMA’s forthcoming move to a new home with a permanent museum.