Shower tub leaking - is this the washer I replace?
"Can anyone verify if this is the likely cause?"


I can't verify but I can tell you that you are on the right track. The flattish washer in the middle is called a bibb washer and (as @Tyson's comment relates) pushes up against a hole in the faucet through which water flows.

The "hole" is not just a simple hole, it is actually a (normally) removable faucet seat which is also replaceable/repairable, but likely requires a special tool to remove.

The rubber bibb washers get a rough groove worn into them, break apart, or become soft over time, compromising their ability to hold back the water flowing through the hole in the seat.

The pic below is probably nothing like your faucet, but it does illustrate how a "bibb" style valve functions (in the pic they call the bibb washer a stem washer- lame):