Thanks for the update. That very well may be true about the sort order defaulting to order of appearance when the custom field is blank*. - certainly makes sense. Would be interested to know what the output is once the numeric codes are entered into the targeted references’ custom field.
In the meantime you could adjust the bibliography sort order by deleting the custom field then reinstating it when you’re ready to do a test-run.
Updated 4/6/2017: For all other remaining records, insert the number “1” in the custom field.