When I bought the Duelund CAST-PIOs for my Carys; I asked which lead was the outer foil. "Doesn't matter!", they said.
I've had a habit of testing everything for noise/best performance, before committing to an install, for decades.
Just an FYI (If one doesn't own an O-scope): Just rig an RCA lead with gator clips and plug it into one of an old receiver/integrated's PHONO inputs, with the volume all the way down. With a speaker attached, of course.
Hook up the cap's leads and LIGHTLY grip (especially films) the sides of the cap, while slowly raising the volume. Keep it real low!
Change the component's input selector, and switch the cap's leads (don't move the volume). Switch the selector back to PHONO.
One direction (when the outer foil's lead is connected to the Positive Phono lead) will result in much more hum.
For the lowest noise in a signal circuit; connect the outer foil to the lower impedance (signal source).
OH, and: It REALLY DID matter!
More info: https://www.aikenamps.com/index.php/where-to-connect-the-outside-foil-on-capacitors
