Checked this out further tonight. Pulled the adaptor plate off which does have a spigot that goes back into the can and appears to go pretty close to the seal in the front of the booster, though I didn't actually check if it ws resting on it or preventing it from coming out of the booster. Anyway, by co-inky-dink, or just typical Datsun/Nissan engineering, the end of the 7/8" M/Cyl is near the same diameter as the spigot on the adaptor and will stick thru the adaptor almost the same length (1mm shorter by my measurements) and I've gotta find my measurements again, but I'm fairly sure when I checked it before, the 7/8" M/Cyl has approximately the same depth for the push rod from the mounting face as the old 180B cylinder. The drama before was just the length of the snout behind the mounting face.
So even if the adaptor spigot is acting as a seal retainer, a ~1mm washer will sort it out.

N.B: I have no idea what the part number is of the 7/8"" M/Cyl I have, but it was supposedly to suit a 240K.

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