Hong Kong SGD27a(w) 1976-78 50c chalky wmk INVERTED block 4 U/M

Postage Due. 1976-78 50c deep dull blue wmk Multiple Crown CA diagonal chalk-surfaced paper variety WMK INVERTED lower right corner block of 4 with Walsall imprint unmounted o.g Very scarce and especially desirable in this positional form. Listed but unpriced in the 2015 Hong Kong sectional catalogue. Originally purchased from Stanley Gibbons (still on their card)