I really need to dig around at home and look in my copies of Big Secrets and Bigger Secrets and re-read this section. Poundstone may have actually written something akin to “more exclusive clearances” rather than “higher clearances”.
Also, IIRC, Poundstone refers to his list of compartmentalized clearances as “in recent use”, which suggests that the clearance names he was passing along were no longer in use even as his books went to press (1983 for Big Secrets, 1986 for Bigger Secrets). “Canoe” could well date back to the late 70s.