If the back end appears to be "slanted" it's commercial. Here's an example:
HECKLER & KOCH HK 416 FACTORY BUFFER TUBE MR 556! : HK Parts at GunBroker.com
You can see the back end is at a bit of an angle. The Mil-spec tubes are flat and square. I'm not sure about the MR223, but ALL of the MR556's are milspec (despite the title of that auction)
The HK buffer system will work fine with most Mil-Spec buffer tubes. I have been using 416 uppers on Colt lower for years and there are countless members here using HK uppers on non-HK lowers sucessfully.
I don't know about your ASAP sling plate as I don't like that area of the receiver as an attachment point, I imagine you'll have to disassemble the receiver extension (buffer tube) assembly. There ARE some other aftermarket attachments where you don't have to disassemble the weapon. If you do go this route, I have a post you may find helpful as it involves removal/reinstallation of the receiver extension:
MR556 Take-down pin removal