These are William's temples. He used the sample end off the last towel warp to make them. They are just very basic drawstring bags that he filled with pebbles (rice or sand would work also)
He uses bent paper clips to attach them to the edges of the selvedge. They help you keep a square grain in your fabric and also make it easier to keep your selvedges neat.
There is a corresponding one on the other edge.
Check out the information at The Woolgatherers. She has a wealth of information including this page on homemade temples. She does some really lovely work, especially in linen
Very Many Active Leggings
11 hours ago