To clarify, here is the "B" side. It's a common cancellation, SG Type Z4 dated Apr 8 87, on a 1/2 anna stamp of India. Stamps of India were used in Zanzibar between 1875 and 1895, when the country became a British Protectorate and issued its own stamps. The use of the 1/2d duty suggests it's either local postage in Zanzibar or, more likely, local post to Mumbai.
Seven basic canceller types were used in Zanzibar, and there are a number of sub-types within these particularly in SG Types Z6 and Z7. I may entertain you with these one day
Looking forward to any help you can give me!