Pretty sure AP means "artist proof." Usually the artists will sell a few extra prints on their own sites. Sometimes prices are a bit higher than Mondo and the prints can be just as hard or harder to get. Phantom City Creative tweeted that they will be doing this with the Avengers: Captain...
FWIW, I picked up GH5 this week and inside was a $5 coupon for other GH titles (some past, some future). Used it on the GH:WT guitar set to bring it down to about $17 bucks. Going to flip the game (already have it) and keep the guitar.