Wartości nominalne monet (w zł)

Próba (stop)

Wymiary (w mm)

Masa (w g)

- https://www.nbp.pl/banknoty_i_monety/pliki/emisja_banknotow_...

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On modern coins, an inaccurate weight, or incorrect alloy (revealed by specific gravity testing) - https://isn.page.link/p1gK

Money of a bad alloy, as well as clipt money, was to be received - https://isn.page.link/wtRU

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The term “improper alloy mix” can apply to two related errors. The constituent metals of a solid-alloy coin can be well-mixed but in the wrong proportions. In the case of a copper-alloy cent with too much tin or zinc, this will result in a brassy color. Alternatively, the constituent metals can be in the right proportions but poorly mixed. This will leave the coin with light and dark bands. Of course, the two mistakes can occur together. - http://www.error-ref.com/improper-alloy-mix/

What Is a Proof Coin?

Before every great work of art, every architectural masterpiece, every brilliant song, first there is a plan, a blueprint, a design. The same preparations for coins have occurred in mints across the globe for centuries. Governments have historically tested out their coinage first through "trial run" samples, called "proof coins" (also called “specimens” or “prototypes”). These proof coins did not enter mainstream circulation and were often used to catch any errors, which were then corrected before mass minting. Proof coins are now considered among the best in coinage.

History of Proof Coins
Since the beginning of coin mintage, proof coins were simply the first ones to go through the press: if one version of a coin wasn’t up to standard, it was corrected until it was deemed satisfactory for mass minting. Because historic proof coins often contained errors that were later corrected, their rarity and unique distinctions make them desirable for collectors today.

united states mint history
The United States Mint temporarily stopped proof coinage during World War One to prevent wasted metal. By the 1930s, the Depression caused a lot of the big changes to U.S. currency: economic hardship forced the government to remove gold coins from circulation while also creating “bank holidays” to prevent a run on cash. In 1936, the Mint reinstated proof coins in order to make sure that entire batches did not turn out ruined. When they did so, the Mint also began selling proof coins directly to collectors at a premium. Today, proof coins are recognized as being more valuable and rare than circulated coins.

Proof Coins Today
Today, because of the element of rarity, proof coins have evolved into the highest quality coins that a mint can produce. They are no longer used to catch errors, but to showcase the best in coinage, and are sold usually in limited quantities with guaranteed purity and a certificate of authenticity. Proof coins can be added to a numismatic collection, a precious metals IRA, or other precious metals investments because they are certified as high quality investment coins—unparalleled in brilliance, luster, beauty, purity, and craftsmanship

https://www.sbcgold.com/investing-101/what-is-a-proof-coin/