I've delved into this a bit. Someone over at AA provided me with a procedure to find the outer foil of the K40Y-9 as well as other caps. I confirmed what he said about the K40Y-9 being inconsistent in its markings. The markings on the K40Y-9 do not have anything to do with which lead is connected to the outer foil – some of the K40s I checked had the outer foil one way, others were reversed regardless of the orientation of the markings on the body. They're random.
Dynaco did use capacitors with the outer foil marked, and, from what I have seen, they were consistent in how they oriented the outer foil. Exactly why they did this I do not know. Maybe there is a measurable, though perhaps not audible, difference in noise. Other than any question about noise, some claim to hear a difference in the sound quality depending on which way the outer foil is oriented. In the event there is a difference in the sound qualities, I think it is reasonable to at least keep the outer foil oriented consistently from channel-to-channel.