Dual marking is allowed as long as the Carbon content meets the "L" quality and the Mechanical properties meet the 304 requirements.

Exactly.
What you will find with these older grades is that the actual properties are way above the minimums.  It is easy to make 'L' C levels and still pass the straight grade strength.
Two notes:
1. You bought this to a spec that doesn't require mechanical testing.  You have no idea what the strength level is.  There is no ASME SA269 because of this lack of testing.
2. Nitrogen does not need to be reported.  This makes 304, 304L, 304N, and 304LN interchageable in most cases.  While N does raise the strength of the alloy it doesn't improve the creap resistance.

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Corrosion never sleeps, but it can be managed.
http://www.trenttube.com/Trent/tech_form.htm

ijzer & EdStainless,
Thank you both.  This is very helpful.  I suspected something like this but when I saw the dramatic difference in mechanicals, it resulted in a big question.

Ours is a heat resisting application with essentially no load so I will be looking close at the other thread that talks about better cost choice for heat resisting app's although in this instance, the quantity is minute so cost is actually a moot point but I need to understand for future reference.

Thanks again.