No, I believe the modern term is ancilliary duty, ie jobs unrelated to the main job. Secndary duties, OTOH, are jobs that are asociated with the main job.
In small terms OC Stn Audit Board would be a secondary duty for OC SSS but PMC would be an ancilliary duty.
In the present context Scotland would be a scondary duty with Defence as Ancilliary, given the vital defence/industrial infrastrure there. These both fit well with his primary duty.