I was hoping that someone who knew what they were talking about would jump in and answer. Alas, you have me instead, although maybe this will prompt someone to give a correct answer.
I believe that the brass spots used in leather working are different from the brass tacks that may be hammered into wood. They have tabs on the back that are indeed bent over, but they aren't long and sharp like a tack would be. They may look like brass tacks on the outside, but are actually different items. Below is a photo off of the internet.
Am I correct in my understanding?
CC Griff
Griff, I've seen "harness spots" like those in catalogs shortly before 1900, but not in any of the few pre-1890 catalogs I've paged through. Chuck Burrows said they came into use on a limited basis in the 1860s, but I don't know his source for that. I've not seen them prior to sometime in the 1880s.
Historic use of brass tacks for scabbards and belts appears to have depended on the tacks and the thickness of the leather. The tacks were just re-purposed upholstery tacks of the period.
Old-style cast brass tacks (one piece head-and-shank, with usually square or rectangular shanks) were hammered through the leather and folded over, or clipped and hammered flat. If the leather was thin enough/shank long enough, they were sometimes clinched: the tip bent, then the nail bent and the tip hammered in, like a doornail. Most of the Indian stuff I've seen them used on, the shanks were clipped and bent over, then hammered flat.
When the steel shank tacks came in (1860 or so, IIRC), they were clipped and bent over. I've only seen one (pre-1890) piece on which they were clenched. Post 1900, clenching seems more common.
When I use steel-shank tacks, I tend to clinch them if the shank is long enough (if not, I bend them over). In order to do this, you sometimes have to anneal the shanks. I set the heads in wet sand, then hit the shanks with a torch til they are red hot. When they cool, you have dead soft shanks on tacks that need to be polished.
If you use plated tacks, they tend to discolor the leather or wood over time.
There used to be a source of cast tacks, but they've stopped carrying them and were unable to suggest a source. Back in '86 I bought >10,000 steel-shank brass tacks. I traded off a couple thousand, and in the years since have used up my stash. Past 18 months have found me buying supposed "brass" tacks, only to learn that the tacks were plated. As soon as the order arrives, I'll give a report on the tacks from the source Abilene found (thanks, BTW--I'd not seen that one!).