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Your test has told you absolutely nothing past the fact that the surface is silver -- as one would expect in sterling, coin, or silverplate. As in books and looks, it's what lies beneath that counts.
Acid testing on silver is not very reliable and always damages the piece.
Just for a laugh, try silver testing acid on:
iron
pewter
lead
steel
Any of these can and have been silverplated. You already found the results of acid testing on silverplated Britannia metal.
Cheers, AinA