Karvs

That's a very interesting find. I wonder how rare it is? Eddop might be able to give us more information on this.

I'm sure even back then they could tell the difference between a silvered bronze and a Denarius, no?

The denarius was only very infrequently issued and was of the same composition. Just a small silver washed coin officially valued at 2 to 1 on the (double denarius) 'antoninianus'. The term 'antoninianus' is also merely a modern designation. We are actually unsure what they were called. No doubt something much easier to express in everyday conversation like our word "dime".

Silver issues of this period when attempted simply vanished once circulated. Bad money as they say chases 'good money' out. The real crisis occurs when the issuer refuses to take payments for taxes in it own currency. The silver content being all but gone the tax authorities wanted payment in gold. The later third century was a time of wild inflation and economic crisis after crisis.
Diocletian attempted a total reform with his new denomination the follis. After twenty years they had been reduced to the coins we associate with Constantine and his sons. While still militarily quite strong they could never put the economic system back on track.

Interestingly while the government mints played games with the silver coin they basically left the gold as a pure metal. Other than a reduction in size the gold was still 99 percent pure.

In some ways it sounds rather familiar doesn't it ?
The top one percent play with gold coinage and the rest of us have base metal tokens or paper money that only has value by our faith that the government will accept it.

Bit coins anyone ?

Now there is strong evidence that XXI does not refer to the number of coins of this denomination that correspond to a gold piece (Gold pieces were very rare in in the late 200s, anyway). Rather, it refers to 20 parts base metal (copper) to 1 part silver. There are very rare "XI" coins with about double the silver. Here is an short article on the silver content of "XI" coins from the Numismatic Chronicle:
http://augustusmath.hypermart.net/AlloyXI.pdf
It is technical, but the overall point is the "XI" coins seem to be about 8.8 to 9.8% silver, which is a lot like 10:1. Combined with articles from other researchers, the conclusion that "XXI" means 20:1 is now commonly accepted.