From the shield type cob to the pillars and waves:

About The Potosi Scandal: It did not take long for enterprising and unscrupulous mint operators to realize they could benefit by either miss-alloying or miss weighing the coins. By the end of the decade of the 1640's this became so rampant that a scandal erupted, causing an official devaluation of the currency minted in Potosi. By 1650's , few would even take a coin minted in Potosi.

To alleviate this crisis a royal decree called, The Cedula of 1650, was passed. It called for all the coins from before 1651 from Potosi to be recalled. They were then to be marked with over 20 different models of marks called also countermarked cobs; then The King after killed the assayers Rodas and Zambrano, He replaced them with Antonio de Ergueta, who had to re struck with a new design to identify them as sound currency.

These were to become known as the Pillar and Wave's, variety coinage, but before the final design during 1652 Antonio de Ergueta had to present different designs in order to replace de old design (Atocha Design) Cross and shield, over than 8 prototypes can be found from 1652, these cobs were called transitional.
 

In 1536 the Mexico City Mint became the first mint to produce coins in the New World. The mint primarily used a method of hammering coins by hand, known as Cob or Hammered coinage. Cobs were produced by placing a blank piece of metal "planchet or flan" of the correct weight between two dies, and then striking the upper die with a hammer to produce the required image on both sides. This method remained in use until it was totally replaced in 1733 by the screw press that we know as Milled coinage.

The change took place by a royal order issued in 1728 that authorized the Mexico City mint to strike new milled coins. In 1733 the mint produced more different varieties of 8 Reales than in any other single year. These were classified into four types: Cobs, Cobs struck with "Klippe" dies, Klippes and the Milled "Pillar Dollars."

The Klippe type coin is also known as a Recortado. The rarest of all four of these types of coins minted in 1733 are the Klippes that were struck on cob planchets. They mark the changeover from hammered “cobs” coins to milled technology and 1733 becomes the final year of “cob” coinage production.
Klippe planchets were produced, first by rolling or flattening silver into a sheet by mule powered roller presses. Planchets were then cut from the sheet and weighed. If needed, adjustment cuts or clips were made until the planchet or blank was of the correct weight standard.

Why the need for these Cobs, Klippes and milled coins between 1732 and 1734?
According to an original 18th century mint manuscript, the new equipment in route from Spain that included 3 new screw presses sank off of the Coast of Little Cayman Island. This lack of equipment, and trained personnel resulted in the production of transitional issues during this period. In 1734 the mint discontinued the production of the Klippes and converted exclusively to the striking of milled Pillar Dollars, making 1733 the rarest Transitional year for the Mexico City mint.

One remarkable detail about the Spanish Colonial coinage is that 8 Reales or "pillar dollars", minted in Mexico City, circulated in The United States of America as legal tender until 1857.

Sources:

- The Potosi Scandal - EnRada Publications - Ernie Richards.

- Wikipedia.org Encyclopedia
- Lords of the East: The East India Company and Its Ships (1600-1874) by Jean Sutton
- Honourable Company by John Keay

- Schatten VOC-schip De Rooswijk overgedragen aan Nederland (December 12, 2005). Trouw (Dutch newspaper), p. 3.