I like the mystery card plot, sandwiches, and monte moves. So, spoiler alert: all three of those are in this trick. One thing I wanted to solve though was the "helper cards" thing that we all do for sandwich routines: "here, these are gonna help us out" or "these two cards are my helpers" or "you like sandwiches?"
My solution was to take the sandwich element and use it in a different context, which ended up being this sweet mystery card routine.
You place a mystery card on the table. A card is touched from the deck and you remove the two cards next to the selection and place them next to the mystery card on the table like a sandwich. The mystery card is revealed to be the selection. Can be signed.
Method: start by removing the mate of the 5th card from the top of the deck and placing it face down on the table. The identity of this card is kept secret from specs.
Then as you spread the cards face down to have a card touched, you cull the 5th card from the top. Up-jog the card they touch and secretly load the culled card above the selection as you close the spread. Turn the deck face up to show what card they chose.
This rest is gonna be videos. It’s simple but a biotch to describe in print. Basically it's two monte moves back to back. First a switch out, then a switch in. In the videos, the six of diamonds is on the table, and the six of hearts is the card you culled. Take a break from reading and watch these crisp vids. First at speed...
Then slowed down:
Now you push the out-jogged card into the deck and reveal that it's in between the two sandwich cards.
So, this will need some presentation context. Essentially the card belongs in between those two cards. By removing them from the deck, leaving their selection behind in the deck, and sandwiching the mystery card on the table, you’re reestablishing the location of their card after it was selected. Thus the mystery card is revealed to be their selection because it’s suppose to be in between those two cards. You can pose the old "can a card be in two places at once?"
Also, I usually use a pseudo dupe. But if you have an actual dupe, you should use it since you're forcing the card anyways.
If this is part of a larger performance, it could be cool to use an already signed card or have the signed card reused for something after. But as a stand alone, I think it's too short to have any significance to the signature. Okay, cool beans.
Name drop: I believe the on the table switch is Gordon Bean's. I think I learned it a while ago from one of Bill Malone's L&L's DVDs. Ah, L&L. Those were good days.