Shawn was talking about a the M261 kit. Here is a picture of a Ciener style .22 kit above a M261 and a M261 magazine insert.
Note how the Ciener kit has 2 rails and the spring is top center. This kit is notched, to allow for adding a weight for full auto use to try and prevent bolt bounce.
I understand that there are a few different kinds of M261 kits, but I don’t know about other styles. It looks like one of the others has a single left side rail and the recoil spring on the top.
This M261 kit has a single rail on the left, and a rod on the right where the recoil spring is. It has a buffer, perhaps urethane. It has a spring loaded tang on the rear, possibly to remove slack and keep the adapter forward in the upper.