Hi Selma.
I am very familiar with the term "manila" through my many years of work with MOA-UBC.
Although English sources translate them as shackles, they are actually a kind of thick bangle or more precisely "bracelet" that was usually made of copper or bronze and were exported for the slave trade among other trades. They served as a currency throughout
the coast of Western Africa from at least the sixteenth century. There were specific exchange rates for these against cloth, iron bars, livestock, cowrie shells and people.
In 19th Zanzibar it was fashionable for rich Swahili and Arab ladies as well as some South Asians to wear chunkier and more elaborate versions of manilas made of silver. No doubt this jewelry also served as a show of wealth. Literally wearing the
chequebook.
I have seen archive images of Zanzibari ladies wearing the ornate silver (hollow) versions of the manilas. This is what the reference is to.
Cliff