Thanks for the welcome.
Yes, the upper has to be LR-308 specific, and not the AR-10 pattern. As far as I can see everything MEGA produces is for either their own "matched" lower/upper pairs, or it is the AR pattern with that 45 degree "edge" on the rear of the receiver, not the half-hemishpherical like the LR. Their page for
Had a quick look at Les Baer and LMT and they both make the AR pattern uppers.
As far as MEGA goes their monolithic upper page is down, I don't know maybe there is another site showing their uppers, a downloadable catalogue or something, but can't seem to find anything.