Question:
Can you show a photo of the "Traded for Slaves" sample-card/s? Not only the past-form "trad-ed" strikes me as odd, but also the timing, since the slave-trade was officially abolished in the early 19th century, at least according to British law.

No matter the text on a sample-card, I very much doubt that certain items - beads in this case - were traded exclusively for certain other items, "slaves" in this case. To my knowledge this was never the case.

Maybe the card's name: "Traded for Slaves" was used in the sense of "This beads have once been part of the African slave-trade? That wouold explain the past-form of the word "trade", but would still not reflect that there were no beads traded for slaves alone!

Maybe I am splitting hairs...?

Anyway - should you have a photo/s of this card/s, please post them!