To update this thread in case anyone else needs this information, the o-rings that fit my universal poppets (as well as the pull ring screw-in lid pressure release valve with replaceable o-ring) are:
AS568 Dash Number 008
Actual Width: 0.070" (1.78 mm)
Actual Inside Diameter: 0.176" (4.47 mm)
Outside Diameter: 0.316"
Nominal Width 1/16"
Nominal Inside Dia. 3/16"
Nominal Outside Dia. 5/16"