Is my leaky faucet missing a valve seat?
There is a seat there on the hot side, unfortunately it has been ground flat. Most (not all) seats have an internal hex shape which facilitates removal with a seat wrench (I cannot tell from your pics if yours does).



You need to pull that seat out and change it. Another (less optimal) alternative would be to "grind and dress" the existing seat, in place. This sometimes works OK and sometimes it does not, and requires a special seat grinder/dresser tool.



Found this image of Jameco tub/shower seats (hex, not square, hole to remove with wrench)