Turret mills; Quick vertical part holder.

[Frank S]

Ah-ha Now I've caught you. And here all along I thought that I might be the only one to use those tiny "T" slots originally intended to be used as a place to mount automatic reversing or indexing stop pins or a means by which the liner glass scale DRO system mounted. For purposes other than the manufacture's intended design.
Time and time again those little 1/4 to 3/8" sized slots have proved very valuable to me when I had to make a set up requiring a vertical off table mounting fixture and yes "T" nuts such as those in my http://www.homemadetools.net/forum/double-t-nut-46086
thread offered a means to gain the much needed extra amount of insurance against damaging the table.
Good on you