Summary

This attachment is meant to be used with a regular woodworking speed square and it allows the user to a ruler (or anything straight and flat really) and position it in a way that doesn't move or sag.

It's meant to measure vertical stuff you cannot reliably measure with a ruler or calipers by transfering the height difference to a more easily measurable surface.

As a side note, v1 has a notch that's specifically designed to fit a small 19.2 mm x 174 mm x 1 mm metal ruler and v2 simply has a shelf that can be used as a reference to stick any ruler you like.

In my case I needed to ensure that a small wooden platform I was building was level so I set the marker to the desired height and then checked each spot while working on it untl all of them were perfect.

The jig has a small fixture to ensure that it doesn't slide freely on the square but if reliablity is crucial I'd supplement it with a small clamp.

As usual, the design is public in onshape under the name : Speed square height gauge adapter