This is the 19mm (3/4") U-Scribe Jig and comes in a set of 3.
The U-Scribe Jig is a one-man, one action jig designed to simply slide onto material without the need for any physical fixing. It's as simple as slide, scribe, cut and fit.
Designed to solve the problem of working alone, struggling with long, cumbersome and difficult to hold fillers and fascias.