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Marine grade stainless steel is 316 or 316L - period.
Cliams like "built of high quality marine grade 304 stainless" are blatent lies.
Stainless steel is a very big subject and if you want to learn it, there are plenty of resources on the web to do so. But if you haven't got the time then take the advices of the SS manufacturers - every one of them identifies 316 as the Marine grade.
Parts manufacturers on the other hand are competing with other parts manufacturers who use claims like "built of high quality marine grade 304 stainless" and in a price driven market are using the cheaper SS to compete.
Boat manufacturers use 304 because it will last past their warranty period, beyond that they don't care, in fact they would prefer it to rust.
Easiest test to establish grade is to use a magnet - 304 is magnetic, 316 is not, (except where it has been welded)
A magnet is also useful when buying hose clamps - many have 316 bands but use 304 screws - a fact responsible for many failures and several sinkings...