M has always been allowed and saves money, especially today. L should last longer in a corrosion environment. Most good plumbers rarely used the thinner wall tubing, homeowners often used it to save $.

You can sometimes tell who did the work by how sloppy the solder joints are. Plumbers joints are even and clean and not overheated.

Some plumbers even went so far as to line up the writing on the piping. Something like how they used to line up the slots in screws.