No one has answered why your coin is
Silvered, so I'll take a stab at it. The
denomination of your coin is given the name "
antoninianus" by modern scholars, and they all feature a
radiate bust of the ruler. It was named for
Caracalla who introduced the
denomination originally. It's not known what the ancients called them, but they were worth two denari. Over the years the silver content of these was lowered until Aurelian's reform in 273-274 which raised the silver content back to about 5%, and it is thought by some that the
XXI (or KA at some other mints) mark on the coin reflects that, 1
part in 20 making 5%. This is not a
fouree, but how they were minted, and as far as I know no one has conclusively come up with method how the ancients made these. There are theories of course, but I haven't heard of anything definitive. You can find these with varying amounts of
silvering remaining, and fully
silvered coins can be
had.
Sorry for the long-winded answer to say "they were made that way and official".
Doug