South West Africa SG ZD9, ZD11 1916 underpaid cover from Windhoek to Omaruru with South Africa postage due stamps

1916 (11.11) censored cover (unsealed) from Windhoek to Omaruru, endorsed with 'Bookpost!' h/s in violet and franked by South Africa 1913-24 1d red, tied by '-WINDHOEK' oval ds (adapted from the German 'SWAKOPMUND-WINDHOEK' train cancellation with 'SWAKOPMUND' excised) with bilingual 'Passed by Censor' boxed h/s below. Adjudged to be underpaid and charged 4d (1d postage deficiency + 3d fine), with South Africa 1914-22 1d and 3d postage due labels affixed and tied by Omaruru cds, dated '21 NOV', with arrival backstamp of '13 NOV'. Lightly folded with pencil notations at left, nevertheless an extremely scarce usage. - P267000692