I was talking with someone associated with the Bill Moran School of Bladesmithing, and he was telling me that they had access to several knives that are purportedly from the Arkansas territorial period (i.e, 1836 and earlier). They were interested in testing the silver on the knives to see if it matched up with contemporary Mexican silver (coinage), but didn't have a basis for comparison. Is there a breakdown of the metals by percentage that would appear in Mexican silver somewhere?
Krause World coins catalogs give the fineness of various Mexican silver coins. A potential problem in this case is that I can see the possibility that the Mexican donor coins used may have had a range of fineness for various types, and the metal from these melted together
Perhaps better to test the silver fineness of the knives directly, using XRF analysis.
As a parallel: I have seen Chinese cutlery that had utilized melted donor silver coins for the source for the silver. In that that case, the fineness turned out to be about .850 fine.
I would suggest that if you have access to an XRF, then acquire a few Mexican coins of that era and run your own analysis and do a direct comparison. Sounds like your looking for the trace impurity's that probably won't show up on an official alloy spec of the era considering they didn't have the ability to check those impurity's back in the day.