Thank you for the reply. I had a look; it is definitely an EDL. The cruise is on Princess; a rare and one-off itinerary from Seattle that does NOT go to Alaska; after a day in Victoria it heads south all the way to San Diego (but not into Mexico) and then returns to Seattle.
Funny enough, I was at my mom's yesterday and asked about her naturalization papers (she was born in Perth to an Aussie nurse and handsome US Navy radioman near the end of the war; she's lived here since she was 2), She went and showed me those very papers, which included a photo of herself at 18; don't think I'd ever seen them before.
What kicked off my fear was attempting to book the trip on a third party agency (same price as Princess but some significant perks) and reading the ID requirements they had there...they made no mention of the closed loop exception. But Princess' website was reassuring; I'm going to book the cruise to get the cabin I want now!