, both stamps showing evidence of the cancellations having been removed as part of an official trial, the reverse inscribed "Cleaned once." and initialled alongside; fine and exceedingly rare. An exhibition showpiece. Photo" /> Proof MA-MB horizontal pair from left of the sheet with inscription<I> "Per Sheet. Place"</I>, both stamps showing evidence of the cancellations having been removed as part of an official trial, the reverse inscribed<I> "Cleaned once."</I> and initialled alongside; fine and exceedingly rare. An exhibition showpiece. Photo | Christie's