cover 1821-37 entire letters (4) showing single oval-framed "G.P.O./Ps Pt" d.s. (HG 15, 15a) with (1821 and 1837) and without segments (1826 and 1837), representing early and late dates of each, and 1827 entire originally showing 6a. postage but corrected with a strike applied on a piece of paper and affixed over the original strike