Live in a 35 year old house outside of Boston.

Had a leak in a Copper water pipe that the plumber finally found.

Unfortunately, I forgot to ask him if it was in a hot or cold line, as this might, perhaps, be meaningful ?

Anyway, what surprised me was that it was pinhole leak in the middle of a run.
Not at a joint or fitting, etc. Right in the middle of a line.

Apparently they used the thinnest Copper they could find when they built the place.

I used a caliper on it, and found it to be 0.028, which I guess is a grade M. (outside diameter of 5/8 inch)

But, it still should take the household pressure without any problem, I would think. True ?

The plumber replaced it with heavier wall stuff of approx. 0.038, which is probablyType L


a. 35 years ago, was this (Type M) a "very common" Copper gage they used for household hot and cold water lines ?

b. Is it still allowed, or the Codes prohibits it now all over ?

c. What might make a pinhole leake in the middle of a clear run ?
I guess the pinhole can be considered as a corrosion type of breakthrough.

d. How common is something like this is the thinwall Type M tubing ? What causes ?

Any thoughts on this would be most appreciated.