The canted over Mk2 version BHA 4667, not needed with the single-circuit brake master ...
... 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. The other arrow is indicating another mark like a letter 'O', which seems to be an alternative identifier, not additional. The internals of this type are different to the previous two, something you need to be aware of when buying repair kits. (Moss Europe).
GMC 1007 which seems to be the current part number for a 4-cylinder replacement. Several vendors show this type with a plastic reservoir (Sussex Classic Cars)
But as of August 2015 both Leacy and Brown & Gammons have confirmed that they have the recently remanufactured original metal can reservoir type.
Is this the one that causes problems with the unboosted dual-circuit master? Whilst the main casting and the diameter of the reservoir seem to be much the same as for GMC1007, the cap is obviously much bigger. (Image from Victoria British)
Yet another variant, this time with a fully cast body but a large plastic cap, pictured by several vendors. It's interesting to note that the cap states "Use only DOT3 fluid"! (Image from URO Parts ('Euro', geddit?))
Original GMC 1011 for the factory V8 and also some Midgets as pictured by Leacy MG but no longer available. Note the single groove round the cylinder near the mounting flange as on both V8 masters, not the two in the Moss Europe drawing above:
Current-stock GMC 1011 for the factory V8 and also some Midgets from Brown & Gammons.
GMC1011 installed on Hans Duinhoven's MGB. Enough room to access the cap even though it is an LHD so the clutch master is more tucked into a corner of the engine compartment that it would be on an RHD. Hans's car is a 4-cylinder, so strictly speaking should have the GMC1007. Originally the V8 master was 1.2mm smaller bore than the 4-cylinder, which with a 4-cylinder slave would give a slightly lighter pedal but less throw, i.e. the biting point would be a bit nearer the floor if the remainder of the mechanical linkage at the master remains the same.
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 has to move past the bypass port before any pressure is applied to the fluid. It also shows that at rest the primary seal has to come back past the bypass port to allow fluid expansion and contraction to pass in and out of the reservoir instead. 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, as well as rendering it impossible to bleed.