I see it is not listed in the schematic.
Is there enough left to get a measurement? If it hardened/distorted, should be able to get it down to two, to try. Standard O-rings probably available at your local H/W store. If it crumbled away, some measurements of the groove, should be able to get there.
It won't help, but the Prod/1720/1701 valve internal O-ring is -111.