1806 Wrapper with large circular FREE - DUB in purple-brown

1806 Wrapper to Taymouth Castle with a very fine and clear straight-line ATHLONE in black 38 x 4½mm, manuscript "3/4 oz / 1/6" overstruck by the distinctive large circular handstamp FREE * DUB * in purple-brown (F/K 118), a second strike just overlapping the straight-line.
The paper of this wrapper has a magnificent watermark showing a seated Britannia within a crowned oval.