Glad to hear it's not worse. I once had to nurse an old 6T about 10 miles with almost no oil to get to the nearest garage, a hose had loosened on a long trip and I only noticed when the oil light came on at idle. She survived without any problems, proving how robust the bottom end is. Sounds like you possibly had the pistons nip up in the bores, or a load of crap came in through the air filter. Is it scoring like from grit/sand or can you pehaps see small deposits of aluminium on the bores from a nip-up? Worth checking into as you may need to look at your air filtration? The L means Low, if you had an H it would mean High. This was a method used by Triumph to match pistons/barrels at the factory, so yes probably on standard. It'a while back but I think I used a slimmed down ring spanner on my cylinder base nuts.