I think if a rep says its 904L SS it should be. Apparently quite a few are not, even if it's only the clasp.
I own 3 Gen Rolex and a lot of reps,including 4 with 904L SS. To me the sheen of a Gen Rolex is the first thing I notice. When you directly compare with a 316 SS version you can see a difference
with IMO, 904L looking brighter and whiter. It's hard to see unless you A/B compare next to each other.
That said, I have some Gen Breitlings and Rado that do not use 904SS but are polished to a very bright finish.
Personally speaking, and you funnily enough said it yourself in a way, I don't think that sheen you see on your Gen has anything to do with 904L. 316L can get that same sheen if finished in a certain way.