I have work for a large university and some of the building are very old...seen lots of 3/8 brass...its probably 75+ years old and still working fine....have also seen 3/8 copper run in houses feeding fitures
some of our blds are that age or older some of our blds have remnants of that old galvi drainage system I forget the specific name but there is one we have brass pipe
that's called "fine thread"not even close to ips thread you would solder an adapter on to the brass to convert to copper if you couldn't thread back onto the brass
