Charles IV 8 Reales

The simple fact that an Eight Reale is underweight is not adequate to diagnose it as a forgery. There are several periods when Mexican mints were "notorious" in that regard. However, in 1804 an underweight coin is less likely and a weight as low as 26.2 grams is suspicious. The condition is not mentioned. A coin worn to AG-G would be in essence full weight - just worn down. I certainly would suggest that the coin be photographed and a the picture posted. Many counterfeits are easily identified from scans. Other more recent high tech forgeries can fool you in a picture, so I would recommend a Specific Gravity test and an examination of the rim and fields to look for evidence of forgery. So, unfortunately, there is no 100% foolproof way of remotely diagnosing a forgery from a weight alone.

When you check the edge design on a Spanish colonial 8R you need to know how it was done. The edge was added BEFORE the coin was struck. It was applied using a parallel edging mill with TWO FLAT BAR dies. The blank was squeezed between the two dies and then one die was moved along parallel to the first (fixed die). As the coin blank rolled along between the dies the design was applied to EXACTLY opposite sides at the SAME time. Therefore all real 8Rs will have two IDENTICAL areas of overlap EXACTLY opposite one another. You would be shocked at how many fakes get that wrong. An edge with ONE OVERLAP is a fake. Period.

Another fact to watch for is the priority of edging. If the edge design causes distortion in the dentils near the rim (say from pressure of application) then you have a fake. The pressure of the final strike should distort the edge design but not the dentils.

These clues are in addition to the search for a casting seam, sprue and vent. You don't absolutely need to know how a fake was made just that a coin was not made the way the mint did. That proves the fraud.