In the SCR it does not contact the lower receiver at all. The top would wedge up against the plastic buttstock, and the beveled lower part of the plate should just slide by the lower part of the inside of the stock, where the recoil assembly is located. The recoil rod/spring is on the very top of the assembly, so everything below the top 1/16 of an inch does not really come into play, as far as I can see. The actual bolt slides back and forth on the two rails, but does not come anyplace close to making contact with the plate - the ONLY contact is at the very top, where the recoil spring compresses. The plate looks like it is staked or welded on, then ground down to level. So as long as the recoil does not break the weld, I am not seeing too many bad things happening. Of course, it could possibly damage the plastic inside of the stock, but I don't suppose that a replacement stock would be all that expensive. Anyway, the RECOIL of the 5.56 is taken up by the spring assembly in the LOWER part of the stock/PG area, I doubt the upper part of the inside of the stock ever makes contact with anything. The BC is very short, with a long strut on the end. The strut matches up with the recoil assembly in the lower part of the stock, that appears to be the only contact. The BC does not appear, to me, to ever come back far enough to contact the stock, it is just too short. That is why you can use any upper but must use their BC.
