You have here a very nice spanish colonial 8 reales silver cob, struck by the famous assayer Thomas de Ergueta at the royal mint in Potosi Bolivia during the reign of Spain's King Felipe IV (1621-1665).
Ergueta was appointed assayer in Potosi in 1651, in order to deal with the debasement scandal of the late 1640's that produced one of the world's first global financial crises.
Due to the sub-standard silver fineness and low weight of the cobs being produced in Potosi, merchants around the world began refusing these cobs as payment.
Ergueta assumed his new role in 1651 and that year produced the last of the cobs bearing the shield design.
In 1652 the first transitional cobs appeared bearing the new "pillars and waves" design (and with the PH monogram above the pillars), which was adopted in its final form by 1656.
Ergueta was active as assayer from 1651-1679, making him one of the longest reigning assayers during the 199 years of cob production at Potosi.
And I actually have a similar one with the same date :p (hence this long description from my database)
You have two dates on your coin, which makes it very nice (3 is the top - you are missing the bottom one in the grid)