Hi Jakob - I formerly worked at a museum where we had many of these situations in the collection. Some might ask if you, too have other instances. Professionals have always told me that keeping the way you record things consistent is important for each institution. So I would suggest you see if there is another example of a "collection" of items that has been donated to your museum and how that has been handled.
I found this metadata document (link below) and have referenced it before. You could compare other plans as well and see what makes the most sense for your institution.
http://nationalwomansparty.org/wp-content/uploads/2011/02/SBHM-Metadata-Plan.pdf
That being said I would give each piece a distinct identifying number/letter. If not it would be very difficult to determine if one was lost or misplaced without this. In the museum I formerly worked, the system was to give an identifying number (year.donor#.artifact#). If there were multiple parts to one object for instance a teapot (lid and pot) a letter would be assigned (year.donor#.artifact#.letter). If there was a collection of individual objects that came in together and were related to one another they were assigned a 4th number (year.donor#.artifact#.piece#). So in my case we would have assigned each of these the fourth number. It signified that they were in-fact related as part of a collection within a donation but could stand alone as well.
However they are entered into PP should match how the artifacts themselves are labeled.
I'm glad you asked this question. I'm curious to hear the responses of others.
Sara
------------------------------
Sara Arnas
Grants Manager
Chicago Academy of Sciences / Peggy Notebaert Nature Museum
Chicago IL