I've noticed that all of the CDE capacitors from the late 1960s in those light-green tubular plastic cases I've run across tested pretty much like new (very close to nominal value and low dissipation factor). I've replaced them, at times, for the sake of reliability, based on advice I've received, but I was still curious as to why they seemed to last so long with little observable degradation of performance compared to other capacitors from that time period (e.g., Sprague Black Beauties). Now I have a better idea.
Not being able to find a CDE catalog from those years online, I e-mailed Cornell Dubilier, and, within a day, I got a response with a scan of a 1965 catalog. Unlike many popular capacitors from the 1960s, these have a Mylar film dielectric only; they're not a hybrid of paper and Mylar. The engineer who responded to my e-mail pointed out that these tend to last a long time for two reasons:
The key to this series' longevity is the molded case (moisture resistant) and CDE's "Dykanol Y" impregnation (stopped using it because it contains PCBs).
PCBs were good at controlling capacitor aging and stabilizing dissipation factor, but I wouldn't recommend cutting one open and ingesting the contents.
Here's the scan for anyone interested. It also contains data on the "Budroc" capacitors you find in some vintage equipment.
CDE Type PM.pdf