You seem to equate number objects with symbols whose printed name is
the written representation of the number object.  This is a natural
mapping at best.  Modifying it by adding a prefix of ":" yields just
another natural mapping.  (The latter has the advantage that it
doesn't invent something new, that it also works nicely with R6RS
readers (which don't have the |...| syntax), and that for some eyes
(at least mine) looks a lot more beautiful than vertical lines in
parentheses.)

I didn't say that using a number object like 123 in a library name is
a new convention [*].  Mapping it to the symbol |123| would be.

Yes, if at least one part in the full library name is an identifier,
we have lexical context information. However, the ambiguity of which
library name part to choose would then have to be resolved.  Always
using the last would be more regular.