A Peak Behind the Scenes

The Doctor Who prestige booklet issued on March 26 includes a pane with four Machins, as noted on Ian Billings’ blog and pictured above.

The March issue of the British Philatelic Bulletin, which leisurely made its way to my mailbox a few days ago, pictures the pane on the back cover, but with an interesting addition. Overprinted diagonally in two lines on all four Machins are the words, “PLACEHOLDER ONLY   DO NOT PRINT”. The four Dr. Who stamps on the pane do not have overprints.

Obviously this is some sort of preliminary image, but does anyone know anything more about it?

--Larry