Pertinax,
Your question sent me looking into brass upholstery nails and their history. That led to brass nails in general, which have also been used on saddlery and other leather products. By far the most common use for brass nails has been in the upholstery trade, so I'll just deal with that. [I've learned that these are called brass nails not brass tacks, so will use nails to describe them).
Earliest use of brass nails for upholstery seems to have been in France during the 17th C. Thomas Chippendale brought brass nails into high fashion in the mid-18th C, with his hallmark "nail trim." The following picture comes from the Metropolitan Museum of Art, NY and shows a beautiful Chippendale chair with a decorative row of brass nail heads. This was considered elite and prestigious during the Regency period in England. It was not until the Victorian period that similar brass nail decorations started to appear on items for the middle class. Thus, common use of brass nails in upholstery seems to have become established around the mid-19th C.
To what extent this general availability of brass nails in Britain/Europe in the second half of the 19th C may have influenced what was available in Africa is hard to say. However, it seems reasonable to think that these brass nails probably filtered through to Africa, via British/European ex-patriots and trade, probably around the 1860s. [I have not found evidence for Asian sources of these brass nails, although some probably existed as the manufacture of brass nails has been around for a long time.] Allowing time for diffusion of these goods to more remote areas of Africa, the availability of brass nails for decorating knives, etc. may have been from about 1870-1880 onwards. This fits with the chronlogy of a Congo knife I have that was said to be collected in the 1880s and had a small number of brass nails on the hilt. Here are some pics pf that knife. Note the dark patina to the hilt, consistent with its estimated age.
Lastly, can we date swords from this brief historical look at brass nails? I think we can perhaps set an earliest date for brass nails decorating hilts or scabbards at about 1860-1870, but that tells us little about the blade which, of course, could be older or younger than the hilt or scabbard so decorated.