I've found out the answer to my question.

All 9 digit Mailer ID's in the USPS Intelligent Mail Barcode (IMB) System start with 9, and range between 900000000 and 999999999.

The resulting number can display

"00 702 923456 789000001 980035555"

but the resulting barcode will be the same.

Therefore, the placeholder number of 123456789 would be an invalid 9-digit Mailer ID, and would be read as a default 6 digit mailer ID.

I hope this helps anyone looking for similar information.