I'm pretty sure it won't. You can find an o-ring that is "close enough" at almost any hardware store. Sorry I don't know the size. But it really depends on the I.D. of the hole in the piston anyways, as they are not all the same.
So funny enough, it looks like it came with the ring. what it did not come with however, was a freaking bench bleed kit. lol. hey, it's here, my car will stop again, and all is right with the world