These are custom designed jaws used to grab a hold of the metal leave-in clips used with fiber cement siding, if one of the clips gets pushed up underneath a plank. These are designed to replace the riveted-on jaws on these $10 Szliyands relay puller pliers from Amazon.
The very rough design was originally prototyped by cutting a slit in some aluminum flat stock with a Woodruff cutter and riveting it to the handles; that was ugly but the concept seemed decent. I then ran through a few rapid prototyping iterations by 3d printing a set of these jaws out of PLA, to determine the final size and form the 10-degree angle for the slot.
The final pieces were ordered from Xometry, laser cut from 0.125" 4140 steel. The plan is to deburr/chamfer if needed, open up the screw holes if needed, countersink one one side (opposite sides for each half of the batch), and then finish them somehow (black oxide maybe?). They'll be riveted to the handles with 5/32" solid brass rivets, peened over on the top side, and the heads countersunk and then milled or ground/sanded flush on the underside.