ISBNs are of limited value since they were only
adopted in 1970. Most
of that material is still copyright protected. The public domain
material on Wikisource is much older than that, and has no ISBNs.
Ec
Once ISBN is implemented we can expand to LCCN, which is everything
copyright in the US library of Congress, and is not a copyright system.
From this other call numbers, provided they can be referenced in the
public domain, will be possible. ISBN's are not of "limited" value,
since anything which has had an edition since then has one. This
includes many texts which were produced prior to 1970, and virtually
all of the commonly cited material on wikipedia. Since the proposed
format allows for different types of identifiers, it will be a simple
matter to add additional identifiers by which material may be cited.