This was left to my Mother by a doctor who worked for a time in West Africa at the end of the 19th century. We had always thought it might be a slave bangle but I sent a photo to the African section of the British Museum and their response was as follows:
"The images of your 'bangle' have been seen by the curator for our West African collections Dr Ardouin who tells me that it is not a 'slave' bangle or bracelet and does not appear to have been produced by African metalworkers. He says that "Grande Bonny " is not a place in Cameroon but will refer either to the Bight of Bonny (or Bight of Biafra) which is shared by Nigeria Cameroon and Gabon or the place called Bonny in Nigeria. It is more likely to be the latter which is an island situated in the southern area of Rivers State in the Bight of Bonny. It is an old city-state which was active in the slave trade and the oil(palm) trade. Bonny traded with the Portuguese and then the British, who were very active there from the 1600s to the 1800s.
Dr Ardouin thinks the object seems to be a publicity medium for a British trader, rather than being a slave bangle, is possibly a ring to hold something, an umbrella perhaps.