Never saw this B4

I did a sewer line repair today. House was built in 75. They had the water line running with the sewer line, common back then. The hoe busted the water line.

The water line was what looked and acted like Soft M not K. It folded like an accordian and was thin like M and dented with just a tap of a shovel. This was a subdivision that at the time was THE place to live, very expensive in it's day. Someone paid the inspector well.

copper

It could not be "M" because that only comes in 20 foot lengths, but a friend of mine replaced his own water line to save money and used ACR tubing. I am not sure if it was 3/4" refrigeration which would have been 5/8" plumbing or if he actually found 7/8", but if I remember right he had some problem making connections so it might have been 3/4" o.d. refrigeration size.

could also be electrolosis. I've done repairs under slab where the pipe becomes paper thin. Is the sewer cast iron or plastic. If it is CI maybe the copper is touching it somewhere.

The sewer was clay, in 1975, thats nuts.