A mounting/alignment gauge is basically a piece of transparent plastic with:
- a vertical reference line on the left side and a near and far vertical lines on the right side to adjust the parallax (horizontal alignment), and
- a bunch of horizontal lines that allow you to align the left and right views vertically.
Note that you don't have to use a mounting gauge to make your own stereo slides. Heat-seal mounts have alignment tabs that make aligning relatively easy ... if you're not to picky (see heat-seal stereo mount instructions).