Great Britain 1867 10d Pale red-brown Plate 1, SG113s.
The 10d value was introduced for prepayment of postage to India, Mauritius and Australia via Marseilles. The rate was changed in 1870 meaning there was little demand for the 10d stamp and it was withdrawn in 1877.
Specimen stamps were used as reference by post masters to identify valid stamps and avoid forgeries.
This sheet of 20 specimen stamps is extremely scarce. It is a very fine and fresh unused pane of twenty lettered FE-JH with full selvedge and marginal inscription at top and bottom, each stamp overprinted 'SPECIMEN' type 2.
The SG Specialised Catalogue Volume 1 last issued in 2011 lists single specimen stamps at a price of £500.