This coin is a neat bullion piece – it comes from a pretty turbulent time in Mexico’s modern history. It has no denomination; the coin is simply marked based on its silver weight (1 onza = 1 ounce of silver). Between 1979 and 1980, Mexico debased almost every circulating coin, so this was a stopgap that also generated profit for the Mexican city mint in the collector’s market.
Most of these are uncirculated, but some were passed hand to hand as merchants gradually began to accept fiat coinage. This one shows some signs of toning, but I think it is in EF45 or higher condition.
On the obverse, the coin shows what appears to be a side view of a smelting crucible, surrounded by the legend “Case De Moneda De Mexico” from 7:00 to 5:00. The date is located about 8:00, written horizontally (while the other legend curves around the inside of the rim).
The reverse shows a balance scale, with the left side tilted strongly downward. It reads “Una Onza Troy De Plata Pura” from 8:00 to 4:00. The purity “Ley .925” is stamped above the left scale at 10:00. Below the scale is the legend “Peso 33.625 Gramos” over the Mexican City mint mark (an M with a circle over the center dip).
Identification code: Mexico M-49b.3
Mint Mark: Mo (Mexico City)
Country of origin: Mexico
Composition: 92.5% silver (presumably 7.5% copper)
Size: 41.7 mm diameter, 3.2 mm thick
Weight: 33.625 (1 oz ASW)
Other details: There are two varieties of the 1979 onza. The Type I is the most common, and it has a slightly higher placement for the word “Una” relative to the other design elements. The Type II has a lower placement and is slightly rarer. Since I don’t have others to compare to, I presume that this is the common Type I (it certainly seems to match all of those that I found pictures for on eBay).
The onza had a very irregular mintage history. It is fairly common from 1978, 1979, and 1980, but was first minted in 1949. When mintage was resumed in the 90’s, the pattern changed and the coin’s composition switched from sterling to 0.999. Those bullion pieces were minted as purely non-circulating coins.
Estimated Value: $38 with silver at $35/oz