Eagle with snake on cactus. Radiant cap with rays, Denomination. Purity of Silver (see comments).Casa de Moneda de México. On reverse it shows the legend "10 Ds. 20 Gs" (10 Dineros 20 Granos); it is a specification of silver fineness used on colonial times. Pure silver was 12 Dineros, and each Dinero was fractioned on 24 Granos, so 10Ds+20Gs= 10/12 + 20/288 = 83.333% + 6.925% = 90.27% = Silver 903.
NOTE: many different varieties in mintmarks. 1 = Real de Catorce (Ce). 3 = Culiacan (Ce, Cn).
5 = Estado de Mexico (EoMo). 7 = Guadalupe y Calvo (GC). 8 = Guanajuato (G, Go).
10 = Mexico City (Mo). 11 = Oaxaca (O, Oa).
12 = San Luis Potosi (Pi).