Not very impressive looking but it had a nice history. And it was not expensive. The last year certainly minted by the royalist government in Mexico. The 1821 issue can be royalist or republican.

This coin has an odd bust for 1820 but I think it is just the small version. The coin should be an official issue, not one of the contemporary fakes. But I believe we have here in the forum people here well qualified to judge that.
Silver surface content tested with a simple x-ray test was 94%. Some surface enrichment as normal? Could not get a test for trace metals.
The edge seems crudely applied but correct.

This small bust version was intriguing and I wanted the opportunity to examine it freely.
I do not think the coin is really much worn. It weights 26.4 g. It looks like the bust of the king was punched deep in the die and suffers disproportionate wear compared to other busts. Or always came out weak on the coins?
The few small bust ones I found photographed all have a weak bust and a weak reverse. Perhaps because of metal flowing to fill in the deeply cut bust?