Chris seems to have nailed it on the head. I'll go further by saying if that ad card code you mentioned for the book ad is accurate, the travel ad card is dated October of '90. I'd wager this deck was printed in late '90 or early '91 and didn't see distribution into customers' hands until at least the spring of '91.
For the Ace of Spades dating code, you need a more up-to-date chart:
http://www.leeasher.com/playground/articles/how_to_date_a_deck_of_cards.htmlI've been helping Lee keep this one updated in recent years. "N" was used in 1910, as you stated, and has hardly been used since - perhaps they wanted to avoid people mistaking it for an M or something like that. Regardless, "N" was revived and used regularly in 2011. Use of the code letter N in the AoS code for the time period of '90-'91 might simply be a previously-unknown one-off or rare occurrence for the period. It's not like things like this haven't happened before. A recent example was sporadic use of the code letter H on decks made in 2013, most of which bear a code letter S instead. Tom Dawson has fondly reminded me that the AoS code by itself isn't always the best indicator, because they've been known to be wrong on occasion - USPC doesn't always follow its own apparent schedule about which letter gets used when.
I will point out, though, that it's become a lot simpler since the move to Kentucky. USPC now adds a four-digit prefix to the letter which indicate the week and last two digits of the year the deck was printed. For example, if a code of "2611-N4593F" was found on a deck, you'd know it was made in the 26th week of 2011, right around the end of June or beginning of July. (In reality, though, there's a good chance you'll never see that exact week/year combination - just as they take a two-week break for Christmas and New Year's Day, I seem to recall USPC shuts down operations for two weeks around the Fourth of July annually as well, meaning they're probably closed on the 26th week of most years.)