[EDITED TO ADD: The API is now fixed and returns the external_id as expected]
The Podio API that returns the diff between one revision of an item and another is documented here
The diff that is returned by the API contains an array of changed fields. For each field we can see the field_id and the label but not the field's external_id.
Is this a bug in the API? The official Ruby gem for this API has a property external_id that is never populated.
Is there a way of getting the field's external_id without making further (rate-limited) API calls?
The Podio API has now been updated to include the external_id of the field.
Just for the record, the Podio dev team implemented and released this fix within 24 hours of understanding the problem. Yay them!