this is from deep inside my old, foggy brain, so keep that fact in "mind" [get it ? ]
anyway, i somehow "think" there was a shoulder on the master cylinder hold down bolt that allowed it to spin and tighten the lid on the master cylinder, and when the lid was removed for servicing, the bolt and retainer stayed together. this made the bolt hard to loose, and insured the correct length bolt was always used with the hold down.
at one time, i had a couple of those style masters around, and they may still be here, but i haven't seen them for years and years.
and if i are "rong", well, just chalk it up to a blabbering old geezer.