I’m building an slr and i just got the gearbox in to finish everything up. I bought an SLR receiver and an x-power buffer tube both from zhenduo, and i can’t get the buffer tube to lock on to the receiver.
All my SLR receivers have a M5 nut on the inside of the buffer tube screw hole, usually the buffer tube screw goes into the spring retainer of the gearbox though.
Try using a longer M5 bolt, that plate is used to clamp the buffer tube onto the receiver inside too if you didn’t know.
No idea why they send that other threaded insert bit, never seen them in a receiver.
Yeah those are the ones I have too, thou mine had the nut installed, the SLR gearbox spring retainer is threaded too from memory so you may just need a longer bolt, although that bolt looks long enough as is.
You put the bolt thru the plate, that goes into the tube and thru the back of the receiver and threads into either the bolt or the spring retainer.
I sacrificed the nut from my honey badger and got it all secured. I think they expected you to just have a nut and then use the anchor as a spacer because the screw is actually long. Fully screwed in it would push the spring retainer and move the gearbox out of where it should be.
If the bolt was long enough you could thread it directly into the gearbox, would secure the gearbox itself better and also prevent the spring retainer from working loose as it’s being held in locked.
If you’re lucky the nut and the spring retainer can be aligned in a away that the bolt will thread onto both.
That extra bit they included is a insert meant to be pressed into something, like a heat insert but without the heat, no idea why they would include that, never seen them used in a blaster.
I concour that’s like a riv nut which I’ve never seen either for blasters.
Should be as Dchapo described screw goes through that washer plate to lock the buffer tube to the receiver while the screw threads in to your spring retainer.
Got me scratching my head. Chuck that riv nut in the bin, problem solved 
I’ve bought X power buffer tubes and they did not come with that nutsert. May be a newer addition to accommodate some other types.
I’ve bought a couple of buffer tube kits with that nutsert in them before.
I used one once epoxied into the back of a spring retainer so I could keep it horizontal with a screw and flat washer… but that’s about as close to whatever it’s intended purpose is that I can get. 
After a couple hours of trying to make it work, i ended up taking the nut from my honey badger and putting that in place, and using the piece as a spacer cuz that screw was longer than it needed to be.
