Of all the stupid ways I've seen LN authors insert paper money into their fantasy world, this one ranks up there as amongst the dumbest. As another user has pointed out, one note would literally be equal to one silver coin so this is completely pointless and only serves to devalue the royal crest as a symbol of authority. The issue wasn't people using counterfeit money, it was people melting down the supply of coins so they could make a profit from the market value of copper. The easiest solution to this is just adjusting the value of the copper coins to match their market value in relation to silver coins, i.e. you only need 500 copper coins to make a silver coin, but the problem with this is that the market value of copper (and all metals) is going to fluctuate based on production, mining discovery, industry innovation and coins in circulation. An actually intelligent way of solving this problem would be using magic in some way to create a kind of stationary device/crest linked to the reported production of the material resource, as well as the current amount of coins in circulation. It would then constantly update each merchant/shop with the true values of each metal in relation to each other as currency.
Do we have to acknowledge the sheer scale of how many magical royal crests would need to be made for this plan to work? If it were so easy to make, then why isn't anyone else able to counterfeit the crest on their own? For how much power and deference someone gets for having one, it would seem to be a high priority for criminals or spies to attempt to replicate it.
I wish LN authors would just stop trying to pretend like they have any idea how economics works.