[fernando]

Your'e welcome Richard ...
I know my theory had a couple flaws, but those hopefuly not irrecuperable.
Manilhas (from the Latin manicŭla by the Castillian Manillas) were inherent to a pre-monetary system used in Sub-Saharian Africa, which included objects like spiral rings and crosses of different weights, where manilhas apparently played a greater role in trade and trafic of slaves since the XVI century in West Africa. These were made in a bronze league, and were produced in England, France and Germany. Portuguese, not exploring copper mines, had to acquire them in cities of those countries.
Similar to bracelets in shape, a most appreciated adornment among African population as a symbol of status, wealth and power, these became a widespread object of exchange in the commerce between Europe, Africa and the Americas.
Existing sources refer the amount of manilhas used in the trade (rescue) of Slaves in Africa. According to the Esmeraldo de Situ Orbis, by Duarte Pacheco Pereira, a slave provenant of Benin in the beginning of the XVI century could be traded for 10 to 12 manilhas. A few years later, as referred by the Regiment of São Jorge da Mina (currently Ghana) a slave would already cost 40 to 50 manilhas. As slavery trafic increased, traders needed to go further into the interior to find them, which spent longer time and cost, their value in Benin, Mina ou Arguim posts increasing circa 500%.
We can read in Rick's excelent link that bronze (and not only) metalwork has developed among Benin artisans long before Europeans (Portuguese) went there, with the slave trade being one of their "hobbies". And potentialy (surely) quoted metal implements were already used as a trade currency.
On the other hand, between th XIII and the XVI century, artisans had to acquire raw material in local resources in order to built their sculptures.
So the 'modern' manilha appears in the scene admitedly with a dual use. Those showing up by the thousand (vide Whydah) with a standard basic shape, with no other plausible purpose than that of slaves 'acquisition', expectedly having a further use; natives would not be supposed to hoard zillions of dull copper rings in their attics. So a part (most) of those would ended up on melting pots for the makng of the famous Benin bronzes; something we may deprehend from from Rick's linked article. The other use of manilhas would be that of resourcing the more decorated ones, more, i would say, for keeping in wealth chests, wedding gifts and so. The example i offered my daughter, despite being the basic version, shows well how these apparent bracelets are not necesarily wrist implements.
Attached examples of slavery imprisonment kept in the Portuguese Museum of Archaeolo and, imagine ... cowrie shells,the longest and most widely used currency of all times, a disputed means of currency in the period.

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