That's what the difference is between 813 and 813a - the silver fineness. It is unfortunate that Britain chose not to put any kind of distinguishing mark on the lower fineness coins. 
I don't think there's a truly reliable way to tell the difference, short of using an X-ray spectrometer, but subjectively, a circulated coin of .500 fineness should appear "greener" than a similarly aged .925 fine coin, dur to the higher copper content. And the one you've tentatively labelled "A" does indeed look greener.
I don't think there's a truly reliable way to tell the difference, short of using an X-ray spectrometer, but subjectively, a circulated coin of .500 fineness should appear "greener" than a similarly aged .925 fine coin, dur to the higher copper content. And the one you've tentatively labelled "A" does indeed look greener.
Don't say "infinitely" when you mean "very"; otherwise, you'll have no word left when you want to talk about something really infinite. - C. S. Lewis




















