> What is for a, b, c, by the way?

Sorry to prolong this, but I don't see an
answer to this part of the message.

/n/a, /n/b, and /n/c are used by a:, b:, and c:,
which now mount on both /n/_: and /n/_.  
the colon-less mountpoint is easier for acme
and the plumber.