... or the later dual-circuit master (image from Steve Shoyer).

... but definitely was with the early North American dual-circuit (image from Dave's Cars).

AAU7152 with the two concentric rings indicated on the cylinder body on the mounting flange side. Not sure what the other arrow is indicating (image from Parts5)

GMC 1007 (image from Sussex Classic Cars)

The offending item? (Image from Victoria British)

Showing how fluid fills the space between the two seals when the pedal is operated. If the secondary seal (nearest the pedal fulcrum) leaks you will get fluid running down the pedal, but the clutch otherwise operates normally (until all the fluid is lost!).

Showing how the primary seal must clear the bypass hole to allow fluid expansion and contraction to pass in and out of the reservoir. If this doesn't happen and the fluid expands, pressure will be applied to the release bearing which could cause the clutch to slip and will definitely accelerate wear.