I do not currently have the time to go into detail.
However, the only sample card that I know of which has a specific not that says that the beads were 'traded for slaves' is a sample card from the Levin Collection.
In his study, Karlis Karklins has given a detailed review of the Levin collection.
The article/book, including a black and white picture of the sample card (page 34) can be downloaded at this link.
At the moment, without a detailed look, it seems that it was more a description instead of an actual indication that these beads were only for the slave trade. The cards are dated between 1851 and 1863.

Please note, the file is large and may take some time to download.