What process is FIRST applied, the precancel or the perfin? Is there an established protocol about the intended use and application of both procedures? Chicago and Detroit are the primary cities on pre-1945 US stamps in question.
Petert4522 is correct. The Precancel device is applied by the post office first, then sold to the particular company/individual who has a Precancel Use Permit filed with the issuing post office in point, then the Perfin is applied back at the office.
Thank you, Pillar of the community. I believe I understand the system: purchase of the precancel and then the application of the private business perfin, and the final mailing process. I appreciate the basics, not having that as a specialty area, but definitely setting aside US and foreign perfins, including the less common US. precancel and perfin combination items.