1840 Mulready Letter-Sheet Forme A 21 with Maltese Cross

1840 Mulready Letter-Sheet Forme A 21 used from Carlow to London with brown-red Maltese Cross, reverse with "open" double-ring cds of Castledermot (Co. Kildare), double-framed Dublin "Diamond" in bright red, and with London arrival cancels in red. The handwriting on the back flap above "POSTAGE" is not file docketing but rather a postscript from the writer. A single filing fold neatly transverses the address panel, otherwise fine and clean. Ex Col. B.B. Zellers.
This Mulready shows the correct intended use as a letter sheet in its own right, rather than as a wrapper, which is so often the case.