The second last line is syntactically malformed and should be,
I think:
: especials = "(" / ")" / "<" / ">" / "@" / "," / ";" / ":" / "\" /
You are correct. This is probably a typo in the troff source.
My preference would be to fix this when MIME advances (or attempts
to advance) to Full Standard, and issue a new RFC at that time.
Keith Moore