With that version of <label> there's still an unnecessary ambiguity with <IPv4address>, it can be solved by <toplabel>:
No, there is no need for that. Every <label> has to begin with ALPHA (and that was in RFC 1035), so if you see anything with just digits and "."s in it, it is either an <IPv4address> or it is a syntax error.
3com killed that piece of RFC 1035 many years ago. These days, 121.net is a valid domain (belonging to a Hong Kong company).