Depends on your definition of "rivet." The short ones are more of what we think of as a rivet - something that holds pieces of leather, fabric, etc. together. The longer, copper one is more of a fastener used on thicker and possibly "harder" material like wood for a trunk. We dug about 20 of them the other day - all with 2 washers - in one small area. There were more but we quit and moved out of the area. Most likely a trunk or some other small box that was put together with them and had long since rotted away.
Daryl