I agree, such devices were not unique to slavery use. It's a bit entrepreneurial to advertise them as such. Saying that they are slave shackles, true or not, brings more percieved value. Just as likely, perhaps more likely, they were used on some other type of prisoner. It's such a basic design and could hail from anywhere middle ages to nineteenth century, local blacksmith-made. (Of course there were blacksmiths on plantations, many of them black slaves themselves, something escaping slaves might have to avail themselves of).
btw such a device as this one in the OP would limit a slave's ability to work, so such would only be used for transport or punishment. A more typical slave shackle would be a long one (3ft or so) for the ankles so the person wearing them could work in the field. That type most often had an additional ring in the center, through which a longer chain could be run through to connect several workers together for walking to another work site. We're talking chain gang, again not just a slavery setting.
Bringing the