Class 125 fittings are made from cast iron
Class 150 black or galv fittings are made from mallable iron
Class 300 (ARR) black or galv fittings are made from mallable iron
There are high pressure fittings, say 3000psi that are made from forged steel.
2 years ago my plumbers came across two 1/2" black caps that wouldnt hold a pressure test. Both of them were weeping very slightly right in the country of origin emboss on the end of the cap. I saved the 2 caps, I was hoping for them to come across a 3rd but so far it hasnt happend.
The 2 caps I have , one leaks in the U, the other leaks in the S. Sure would have liked to complete the set.