The welded doesn't have anything to do with it. In fact I'm pretty sure all of the major Mills (in the US) that provide Sanitary tubing are all laser welded. They polish the inside to make it seamless though making it an A270 spec. but technicall it's still rolled and welded. Some provide seamless tubing but it's usually electroploshed for seriously clean and sensitive applications like semiconductor plants and such.
I think I found it.
I think it's basically that site and those "304" nipples in general? A lot of pipe, tubing and nipples will be dual cert 304/304L
. The L stands for low carbon. 304 carbon MAX is right around .08 while 304L is .03. 316 and 316L is the same exact amount of carbon but a higher Nickel %.
From the pictures it does look like they are stamped but I can't tell if it's dual grade. Either way you're good and don't be scared. Here in Phoenix there are TONS of food processing plants that use 304/304L. Possibly your bottled water you're drinking or eating, gatorade, pepsi, coke, potato chips, subway dough, bakeries, hot sauce and so many more.