Welcome to the forum. You will be a great addition to the knowledge here on the Forum. Especially since many of the high end reps use the A7750 movement.

As far as your question, Rolex started using the 904 steel in 1989 so if your sub was older than that it would have had 316. I personally thought that my Rolex which had 904 seemed to scratched less than my other watches with the 316. Not a huge difference though.