A little coin appears in Seven Noble Knights, once Mudarra has been in Castile for a while. He wants to give money where it wouldn't be seemly, so he mitigates his crime by casting a few obolos into the street. Described as parchment-thin and barely worth enough to buy a loaf of bread, Mudarra's coins are based on something I have in my possession.
I got this little coin on Ebay (what can't you get there?) for little money ten years ago. The vendor told me it was from the reign of Alfonso X el Sabio, pretty much my only reason for being alive at the time, so I couldn't resist. I was later able to verify in a museum in Burgos that this is just like other obolos out there, so I feel pretty confident that it's the real deal.
Of course, Mudarra couldn't have thrown a coin that bore the emblems of both kingdoms because he lived during a time when Castile was an independent county officially still part of León. It's still likely obolos were struck at the time because of the eternal need for very small values of coins. Aside from the thinness and small circumference, one mark of a coin of small worth is that it hasn't been cut to make even smaller values. Most important to me as the author, this coin was minted during the reign of my favorite king in the history of the world and the same king whose team compiled the books where we find the first traces of Mudarra's story.
This coin weighs almost nothing, but I can feel the seven hundred years in the patina. It brings me that much closer to the realities of the lives of my characters.
Happy holidays! See you again from my new home in the new year.