These are regarded as ancient cultural markers of the Fang.
"In a little bay we pass we see eight native women, Fans clearly, by their bright brown faces, and their loads of brass bracelets and armlets, intent on breaking up a stockaded fish-trap." Mary Kingsley, 1895, Travels in West Africa.

See https://boasblogs.org/dcntr/collecting-on-women-in-gabon-during-the-colonial-period/

Tessmann (1913) Abb. 222a. Verzierungen auf Messinghalsringen shows a selection of designs observed on these collars. There is not an exact match for the design seen on this example.