A while back I saw this lathe accessory on thingiverse and downloaded the files, I printed the bits a few days ago and have been fiddling with it on and off, press fitting captive nuts inside it and tapping sections.
It's a handy little thing for 2 purposes, it has a profile that allows it to be clamped to the lathe ways and also has a clampable hole that accepts something that is 8mm diameter. So it's very handy for holding a dial test indicator (as above) for example to make a very precise last cut or other accurate positional stuff.
It's also pretty useful with just an 8mm piece of steel bar in it and the bar can be set to a fixed position and it acts as a stop guide for the carriage. Useful for quickly making repeat passes/cuts on a job to the exact same point. I like the idea and it works ok, I might do a redesign at some point as I think making the assembly a bit wider would be useful and would increase its clamping force making it adhere to the ways more securely.