This item was spotted on eBay recently, the 1st class gold with slits but no Royal Mail security overprint. With a postmark of 23 April 2009 it could have come from ordinary counter sheets (issued 17 February) or the early Business Sheets (issued 31 March).
The pair, which sold for £21 on 16 September, was described as:
The error is a completely missing iridescent overprint covering the whole stamp, this overprint is the Royal Mail in wavy lines which has been completely omitted from these stamps.
It's been suggested by somebody who has found some more, that these are "most likely from the DLR coil tests. There are some oddities but given the post mark date 23rd April 2009 they are from tests run by Royal Mail and have slipped through the system.".
My 50th Anniversary columns for Linn's Stamp News - For my last Great Britain columns for *Linn's Stamp News*, I decided to go out with a bang and summarize the history of the Machins in a format that would...