Guide to the Shield Counterstamp (Brazil - copper):
In 1799 the copper coins started being minted with half the weight, changing the weight to face value ratio established before. In 1808 the Portuguese Royal Family ran from the Napoleonic Wars, when they arrived in Brazil a government economic crisis started to occur. To increase the crown treasure many economic policies took place, one of them being the counterstamping of the older copper coins (from before 1799), doubling their value. Though it's not our main concern now, it was also applied in silver coins struck in the Jose I reign struck for Minas (mining region). These silver coins were struck with 75, 150, 300 and 600 Réis facial value to help the transactions involving gold (they are the "J" coins). Since they weighted the same as higher value coins, the countermark increased their value (80, 160, 320, 640 Réis).
About the stamps:
Returning to the copper coinage, many stamps were made in Rio and shipped to other provinces, but, since they didn't last long, other stamps were opened all around the country, some very rustic looking. Once all the profit from the stamping was supposed to go to the crown, some stamps were illegally opened and applied by some citizens. It's very hard to distinguish the originals from the false ones, therefore it's very hard to spot the modern forgeries. Some even ended up stamping coins from 1816 and other years, some very original looking. The original legal text also mentions that the V Réis coins (that weren't being struck since 1799) were not supposed to be stamped to facilitate the small transactions change, and that the X Réis struck from 1799 onward were supposed to be stamped, but in this case it would cut it's value in half for the same reason previously presented.
About the placing: Almost all the copper stamps were struck over the facial value, in the obverse. When it comes to the silver coins, a good amount also received an extra stamp in the reverse, near the lower edge (there are also copper coins like this, but they seem to be an exception).
It's very important to notice that there are coins from after 1809, silver and copper ones, with the shield stamp, but most of them were actually struck over older coins with the counterstamp. In the early 1820's a good amount of the copper coins struck in Bahia used older coins as hosts, the government officials seemed to know the confusion that the visible shield from the base could cause in the obverse and all (I've never seen otherwise) were struck with the obverse over the reverse, placing the shield in a spot where it wasn't supposed to mistakenly interpreted (I also have a 1816 XL Réis like this in my collection, but I've never seen another one like this). There are also some rare imperial coins (1822-1832) struck over colonial coins with the stamp, in this case the mint didn't take the same precautions.
Some stamped Portuguese coins are known, most are accepted as being from the period (most are rustic looking, seeming to be false, some even modern forgeries).
About the rarity:
It's very common to see the late 1700's coins with the stamp, but as we go back in the century they become scarcer and scarcer. The first coins to circulate in Brazil that received this stamp were the XX and X Réis struck from 1693 to 1699 to Angola and were authorized to circulate in Brazil by the Royal Charter of February 10th of 1704. Most of the stamps over these coins seem to be forgeries, but there are originals reported and photographed since 1908. There's also a known example of a 1820 XL Réis from Bahia struck over a XX Réis (1693-1699) without the stamp, which proves that they were circulating at the time. The post 1799 stamped coins are also scarce, even the one with fake stamps.
Example of a good stamp - copper
X Réis 1784(L)
Example of a good stamp - silver
600 Réis 1771
Example of a copper coin with the stamp on both sides
XX Réis 1729-1731-B
Example of a silver coin with the stamp on both sides
300 Réis 1754-R
Example of a somewhat good stamp on a XX Réis 1699
Example of a XX Réis 1820 B (Bahia) struck over an older coin where the shield appear in the reverse
Example of a 640 Réis 1826-R struck over a 600 Réis with the stamp on both sides - obverse
Example of a 640 Réis 1826-R struck over a 600 Réis with the stamp on both sides - reverse
Example of a 640 Réis 181? struck over a stamped 600 Réis
Example of an 80 Réis (around 1826) struck over an older coin with the stamp
Example of a XL Réis 1816 B (Bahia) struck over an older coin where the shield appear in the reverse
Example of a 20 Réis 1825-R struck over a X Réis from José I (1751-1774) with the stamp
Example of another 20 Réis 1825-R struck over an older coin with the stamp, but this time in the reverse
Hey, hope you liked it! The base for this text was the 1942 Article wrote by Kurt Prober about the same subject. To my knowledge, this is the first text in english about this subject. Sorry for the grammatical mistakes!