The only E-Number that ends in "15" is 10915 (Manufactured by Luxfer and has a 5-year hydro if last tested after May 11, 2001). And I'm pretty sure that Nitroduck has never offered GCI tanks...
I'd email the seller just to verify what the number is, though. It's better to be safe when dealing with HPA.
Call me "Mig," it's easier.