Mexico ESTADOS UNIDOS MEXICANOS 1 Onza

The coin mint called "de balancín" (of sway), was used in Mexico City Mint to make their first coins. This device uses the rotation of a pole on top with weight on a side to get enough speed to force a spiral down and mint the coin (later updated to use steam and then electric motor but the method was the same).

Some people think that these rounds contain more than one troy ounce of silver and are led to believe that they have scored two and a half extra grams of silver for free. The reason that these medallic pieces weigh 33.625 grams (instead of one troy ounce, 31.103 grams) is that the silver alloy used in them is, unusually, only .925 pure. 33.625 grams multiplied by .925 purity = 31.103 grams of pure silver.

Obverse
Coin mint called "de balancín" (of sway) in the center. Mint name above and year to the left.

Script: Latin

Lettering:
CASA DE MONEDA DE MEXICO
1979

Translation: Mexico City Mint

Reverse
Scale as a centerpiece with the weight as legend around, the purity at the top-left, and the weight in grams at center bottom, and mint under

Script: Latin

Lettering:
UNA ONZA DE PLATA PURA
LEY
.925
PESO
33.625
GRAMOS
Mo

Translation:
One pure silver ounce
Fineness 0.925
Weight 33.625 grams
Mexico City Mint

Mint:
Mo Casa de Moneda de México, Mexico, Mexico (1535-date)

Features

Issuer	Mexico
Period	Republic (1905-date)
Type	Non-circulating coin
Years	1978-1980
Value	1 Troy Ounce of Silver
Currency	Silver bullion
Composition	Silver (.925)
Weight	33.625 g
Diameter	41.5 mm
Thickness	3.0 mm
Shape	Round
Technique	Milled
Orientation	Coin alignment ↑↓
Demonetized	Yes