I love the Palm Springs Air Museum. They have a real friendly group of volunteers and having been there for the Mentor gathering the last 2 years, I can tell you they have been very accommodating and fantastic.
Great to hear they are going to get the PBY flying again. They have made a lot of progress in the last year.
The mm number on the inside of your lens cap is the inside diameter of the lens, or the size of a filter (or replacement lens cap) that you can use. That's another reason I don't like to use filters, too much work to shift them and keep track of. And when you are at a show like Chino, you barely have time to replace a flash card, much less a filter. Of course, if you are shooting things that are a little less speedy and you have the time, it's not so bad.