Re: 308 vs 7.62 Nato
NATO ammo is carefully watched to make sure it stays under the 49,700 psi limit while recent commercial ammo in 308 Win is allowed to go as high as 62,000 psi so I don't know how your friend is blaming the NATO ammo being too hot for breaking your extractor. I would more suspect a headspace issue.
I have an M1-A rifle marked on the barrel 7.62x51 made by Springfield Armory that I was a little hesitant to shoot 308 Win in so I called Springfield & got their dealer rep Dawn. She told me that the M1-A/M-14 was designed (as your L1A1 rifle was) for the 7.62 NATO round but that all of their rifles would handle 308 commercial ammo just fine. She also said they are now marking their rifles as "308 Win" to avoid confusion.
I don't know what the official word is on the L1A1 shooting .308 commercial ammo but lots of folks are adjusting the gas down & using it in their L1A1's.