Are you talking about this?
The Chuck Stop by Edge Technology is designed to locate shorter parts beyond the chuck jaws to provide clearance for facing operations. The Chuck Stop uses different height parallel bars to achieve the required spacing. A centering plug is used to keep the unit centered to the ID of the chuck...
www.edgetechnologyproducts.com
I have one, and use it pretty frequently. It's great with thin stuff. Sure beats holding parallels under the part by hand.
I do wish it was easier to swap heights. It's not difficult per se, but it is a bit on the time consuming side. Thankfully, I don't find myself needing to change it that often, I can usually make things work with one of a few sets of parallels.