How to edit a part instance attribute

We are using part configurations and part instances to track serial numbers of high value items in our assemblies.  I would like to have an attribute on the instances for some general notes.  Currently this is a global attribute which I have added to the 'Part Instance' type, it is added to the edit layout, the visibility is set to read/edit, and the policy administration gives the 'modify' permission to users.  It correctly shows the attribute on the item information page.  However, there is no edit option on the action dropdown box on the part instance.  There is no checkout/checkin for these type of objects either.  How can I enable a user to be able to edit an attribute?  Attached is a screenshot of the attribute setup and layout for this type.