I agree that's the only rational conclusion. I wasn't expecting a joker and I was immediately upset that I had a "damaged" 2 of diamonds (much the opposite of someone writing a missing card value on a joker). I flipped through and found the cards in order, complete, with a 2 of diamonds. Whew.
I'm still interested in finding the original joker (or a <> 1902 De la Rue joker from a deck with these faces). I'm still treating this as my own mystery. When I get my carbon-dating results back, a second opinion from the handwriting analyst, and a response to my official query in the Queen's own hand, I'll let everyone know (in the off-topic section).