Andy,
It looks like P3348 is not actually filled out completely and a bit incorrectly to my eyes.
2. Secondly, what is so important to know about the National Library of Greece Authority Records ID? that could all be captured on the property P3348 itself.
- It's an external identifier. We already know that through the authority assumption and ExternalId type.
- there are a few things to say about the identifier, such as what does it really represent? a work? a title? both? broadly any single item in their holdings? ( look at how https://www.wikidata.org/wiki/Q18609040 uses facet of P1269 to hold the representation "taxon" at a higher level for all property that represent a "taxon" like https://www.wikidata.org/wiki/Property:P627 )
- Are the same IDs also held and included in a particular named database that often gets cited or referenced? Then we should say that they are included in that named database with some property (of which I don't know if we have something that works for that currently)
- Are the IDs constrained and only issued and generated for some holding type or set of collections for National Library of Greece, for example, only official active holdings (not dropbox holdings that have not been vetted)
- Is there an email or phone number for the Authority Records questions, then I'd add that also to P3348.
- What else is there to say about the IDs, what they represent, who issues them, etc. Capture all the metadata of the authoritative ids on the ID property itself.
I think your particular question is that around bullet point 3, where an authority might maintain 2 or 3 holding types or collection sets, and then issue sets of identifiers for those 2 or 3. Each holding type or collection set might be in a database that is named and known and often cited and has many things to say about the database itself, like who owns it, when it was first created, etc. Irrespective of all the sets of ID's that it might contain.