I got the Midwest Industries QD Rear Sling Adapter for 4-position CAR Stock installed on my Mech-Tech CCU. This is the BEST
and CHEAPEST way I know of to put QD swivels on a CAR-15 stock. There's no sling loop or stud to use an adapter on, just the slot and top loop. These have an MSRP or $30 but are on sale for $15, when you can get them. They keep running out of stock. I almost bought 2 since they're half price but don't plan on ever buying another CAR stock. An M4 stock is an inch taller, so unless you want something smaller than that, or want to have the right kind of stock on a retro CAR-15 build, why bother? But if you do have one, you can now have QD studs on BOTH sides,
cheap. AND it comes with a QD swivel, so that's one less thing you need to buy.
https://midwestindustriesinc.com/qd-rear-sling-adapter-for-4-position-car-stock/ P.S. It comes with a longer pair of screws too.