I finished this rig last week and it works great. The "set-tru" function comes from the collet chuck's flange bore I.D. being a bit larger than the lathe spindles's nose taper allowing it to be slid around by 4 brass-tipped 10-32 allen screws. You snug up the chuck flange nuts, dial in a perfect runout then finish tightening the nuts. Works a treat and allows dialing in a collet that happens to be off a little or in my case when repairing existing part features that aren't perfectly concentric. I get a bit of small, fairly close tolerance work that's needs to be chucked in a collet but for whatever reason doesn't run true enough. I used to put an ER chuck in my 4 jaw & that worked OK but the overhang was too much for my taste.
The smallest brass I had in the shop for the screws was 1/2" dia. and really didn't want to waste that turning it down to 5/32" so I found some 1/4"-20 brass screws at Home Cheepo @ 2 for a buck & change. Way overpriced but within my budget. The alloy is unknown but machined beautifully and doesn't mar the spindle nose taper.
The spindle nose taper is a bit over 7 degrees so I put the chuck in a spin-dexer at that angle in the mill to drill & tap the 10-32 threads to allow the screws a square fit against the taper.
I really like how it works other than you have to get used to the adjustments being bass-ackward from a 4-jaw.