Does anyone know which field the below would be in a saved search? I’m trying to filter a saved search by the approval process manager approver (the next person who needs to approve a PO) but I’m unable to find it in the criteria list within the saved transaction search. I’ve tried adding every single ‘Approver’ field but all show as un-assigned.

You cannot search on that field as it's a "custpage" field meaning it's being added by a script "on the fly". Try putting the record in edit mode to see if there's a field with a different id showing the same detail that's not "custpage".