Starting with 27 small cubes connected to each other in a 3x3x3 cube shape, I removed some of the cubes so that all the remaining cubes stayed solidly connected by cube faces.
I then used the resulting object as a stamp: I touched one face in ink and pressed it on paper. After doing this for 3 faces of the object, I had managed to spell out the word TOY:
X X X X X X X X X X X X X X X X X
Question 1. (the original question): Which letter can be stamped using the face that's opposite to the O face?
Question 2. The face opposite the T face can also be used to stamp a clearly recognisable letter. Which one?
(nb. The question didn't originally specify that the small cubes had to fit inside a 3x3x3 volume and be connected by cube faces.)