Same here Ian. High grade 1856 OT pennies seem distinctly rarer than high grade 1843, 1849, 1860/59 currency, and 1839 & 1853 proofs. Even at low to middle grades they are hardly ever seen.
I did notice an a/UNC specimen on offer from Dave Craddock a year or two back (for about £1600 I think), but hadn't read until the e mail had been in the in box for a day, and on enquiry, it was, as expected, gone.
All the more frustrating, looking at your chart above (for which many thanks by the way), where is can be seen that they formed over a third of the 1856 total.