From our examination of the plans, it shows the jig tab on the bottom of the stabilizer. If it's properly done, the stabilizer will have symmetrical taper from the root to the tip. One side should not be flat with the other tapering to it.

If your stabilizer has one surface flat, then orient it so that it's on the top. It will be a bit more appealing, visually. Because the airfoil is symmetrical, it doesn't matter which side is up, but it will look better if the bottom tapers up to the top.