I received two 7x15 stainless ports today. When I ordered them I was
assured they were 316 ss. So today I thought I would see if a magnet
would stick on one. The magnet stuck. When I called the supplier about
another issue I mentioned that my magnet was sticking to the 316 ss
port. I was told the polishing process was to blame and the 316 had
"picked up a slight magnetic" charge.

Is that true?

Are there simple NDT methods to determine if a stainless alloy is 304
or 316?
Bob