Note: "1-1/8 inch" refers to the outside diameter of fork that the stem goes into. The actual diameter of your stem should be 25.4mm (1") for this model stem riser.
Made of cro-moly steel.
A stem raiser fits into the steering column in place of your present stem, and secures with a wedge bolt in the normal manner. The upper part of the stem raiser sticks out above the steerer, and is bulged out so that you can insert your original stem. The bottom of the original stem winds up a bit above the top of the headset, so you gain several inches of height.
This size fits bikes with "1 1/8 inch" "oversized" stems. Actual outside diameter of the riser is 1" (25.4) mm. This size was used on some mountain bikes from the 1990s and early 2000s, but it is not as common as the 1 1/8" threadless type used on most newere bikes.
Note, you may need to install longer brake and/or shift cables.
You'll need a long 6mm allen wrench to tighten the bolt in the stem raiser.
See: "Hands Up" by Sheldon Brown