Old Spanish Coin Identification

The legend reads Carlos and Juana placing it between 1536 and 1571. And yes it would be Mexico colonial. By the M P assayer marks on the shield side it would be 1538-1541 I believe. By the diameter and mass, and the pair of dots above PLUS, it would seem to be a pretty underweight 2 reales.

I cannot speak to the authenticity at all, and expect others will chime in on what I believe it is

"Hispaniarum et Indiarum Rex" were part of the titles of the Spanish kings. It literally means "King of the Spains and the Indias". "The Spains" refers to the ancient Roman division of the Iberian peninsula into two provinces, Hispania Citerior (Near Spain, the easternmost third) and Hispania Ulterior (Far Spain, the remainder). "The Indias", otherwise translated as "the Indies", refers to Spanish rule over both "West India" (the Americas) and "East India" (the spice islands in Southeast Asia). The titles appeared often on coins from the Spanish colonies and rarely on Spanish Homeland coins, though on later coins the title was often abbreviated to just "HISP ET IND REX".

The corrosion on the reverse around the bottom of the shield might even be salt-water concretion; a "shipwreck coin". Though with no providence to connect it with a specific wreck, it's just corrosion. It obviously had much more corrosion on it originally, the removal of which which may explain the reduction in weight.