I call these "lap anvils."  They appear to have been used to present a hard working surface that can be stabalized with a hand and a leg.  The top photo is a view of the top working surface of each, and the bottom photo shows the curved bottom of each which sits nicely on the leg.  The bottom anvil in the bottom photo is being propped up with another rock to help illustrate the curve.  These were found in the same artifact field.