The old washer is in the upper right; you can see the indentation from the valve seat.
There’s a variety of sizes & shapes; these represent just the closest matches. I have no idea what 3/8, 3/8R, and 3/8L might signify, but they’re all slightly different, some with conical cross-sections that may also be slightly different. Worst case, of course, you can sand down the rim of a too-large washer to make the diameter come out right.
[Update: a table of sizes mentioned in the comments.]
The washer just in front of the old one has information molded right into the back:
GOLDEN STATE 10¢ 1/2. Now there’s a show of confidence in price stability that you don’t see much any more!
I found one that fit snugly in the recess of the valve stem, turned the screw tight, and it’s all good.