Cape of Good Hope 1859 6d Deep Rose-Lilac on Letter per RMS Celt

1859 Small format envelope with letter content per "RMS Celt" (of the Union Steamship Co) from Cape Town to Sheffield UK via Devonport (26 SP) and London (SP 27). Franked with a 6d deep rose-lilac (SG 7b) triangular paying the mail steamer rate. The adhesive with full margins all round, just touched at right base; has very fine colour for these. Shows red crayon "5" being the 5d portion of postage to be credited to the UK. A VF clean 6d cover. This was Celt's fifth return voyage departing Cape Town on 21st August and arriving Devonport 26th September.