There is actually a Baskin Robins Collaboration with Puzzle & Dragons that was exclusive to Japan. The "サーティワン" string most likely also belong to the puzzle and dragon cluster lol.

If you google "パズドラ サーティワン" you'll bring up a bunch of Japanese sites talking about the (now very old) collab.

An example being this tamadra, holding an icecream: https://www.appbank.net/wp-content/uploads/2014/07/31-3.jpg

There were a total of 7 units all with their name prefixed with "サーティワン"