The Activity Club and the Ice Dungeon Challenge

Isaac and Max walk over to the advent calendar. However, it isn't like any advent calendar they've ever seen! There are only 26 flaps, each marked with a letter of the alphabet instead of a date. A note on the wall behind the advent calendar reads:

"The nutcracker holds the key in its mouth. However, the nutcracker is booby-trapped, and there is a locking mechanism on its mouth. Open the flaps in the correct order to obtain the key.

41-50-61-51 31-81-10-91-90-40

Feel free to use the mirror provided to put things in REVERSE."

Max and Isaac look at each other and nod. Isaac takes the note off the wall and puts it in front of the mirror. Then, upon realizing what the message says, he moves to open the first flap. Which letters should he open first?
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