Yes that would be an o-ring and yes the auto parts store is a good source for those o-rings. You can see the green o-ring underneath the hex nut in this pic. This particular valve is the one on my late 60's edger that I restored last year. The pitcock does unscrew from the assembly and that's how you access the o-ring.
What a great forum - all you gotta do is ask and people are always willing to help! I have O rings, but the remains of the original looked like it was fiber. I thought maybe fuel would eat a rubber ring up, but obviously not.
