Now that the items table has all of the 952 subfields I did a little test and when I put data in the 952f I can't find the value I added in the database - I also clicked to edit the item record again and found that the value was no longer on my item record. My conclusion here is that the 952f is not stored anywhere in the database.
I retested this and it seems to work all ok now. The field is not mapped to a column in items so it's saved to more_subfields_xml. I could add, save and edit the subfield without problems.