I have one of those... Always imagined it was related to the slave trade... But probably too late ...
Here are a couple of links... The first is to a Canadian ebayer with an amazing collection of gins including one similar to yours... The other gives some background to gin in Africa...
It's a British bottle made for the export trade. There is another bottle identical in size and shape but embossed 'NIGERIAN', that was filled by the same London based company. The company name escapes me now but if I remember I'll post it here. These bottles are much later than the slave trade, and date to 1880 - 1900 (the slave trade was outlawed for British vessels in 1807, and from that date any British national involved in the trade was liable to prosecution on return to the UK. Ownership of slaves in british colonies was not outlawed until 1833).
These bottles have turned up in huge numbers in former British colonies in West Africa in the past 20 years, especially Nigeria and Cameroon.
Not sure about value on your side of the Atlantic, but value here in Britain would be about $30 - $40.
Hope this is helpful.
Jerry
There is one guy around these parts that has imported those by the thousands. He has a warehouse full and sells thm for 10 bucks each at local flea markets. Claims they can be found in HUGE piles in S. Afrika.