Copper Spelter is a type of hard solder/brazing material. It's generally 50/50 copper & zinc, but a home-made mix can be about anything that's close to that.
Filings were the source of the materials & mixed together with a flux, usually borax.
But I don't know where the zinc would come from in that early American gunsmith setting.
Something I know little about.
It was used quite commonly in the European gun trade to braze double bbls together at the breech and for other situations where a braze joint was needed.
I think a form of it was also commonly used to cast decorative metal objects like furniture pulls, lamp bases, etc.
Brass itself being a copper & zinc alloy can do the same. Perhaps the 50/50 copper & zinc has some advantage in strength,,, lower melt&flow point maybe,,,just guessing of course.
Maybe the copper in the inventory wasn't even used in his gunsmith trade. Seems they were sort of Jack of all Trades types.