I don't think the air is escaping from the Regulator!!! The Air is blowing from the channel above it!! So It could be an O-ring the seals the valve-rod, or the O-ring the the Nose piece of the plenum...
From the video at first glance it appears to be part E100=19763 on the mk3 which is a 9 x 1.5 nbr 90 located on the power plenum stem at the clamping area. Can't be 100 percent positive but that is my guess from the video not the reg.