Nice little find for $3. It looks to be a makers mark but I cannot make it out even after enlarging my screen to 400%. I've never had a problem with the purity Mexican silver found in my area, especially vintage silver. Half of the stuff I find at sales is usually Mexican (Taxco) and it has all tested good so far. Maybe some of the stuff on the west coast is a little questionable, I don't know. I have run across other stuff marked .925 that was fake but it wasn't marked Mexico. Alpaca is of course nothing but copper nickel and zinc that is sometimes silverplated.
One thing to keep in mind- If you ever find any Mexican silver marked "plata," that means silver in Spanish, not "plated" as one may first assume.