The 1909-O Indian Head gold $5 half eagle has the lowest mintage in the series, with 34,200 pieces having been reported struck at the New Orleans Mint.
Coin Values prices the 1909-O half eagle at $7,000 in Extremely Fine 40 and $11,500 in About Uncirculated 50, making this date and Mint mark extremely interesting to counterfeiters and alteration experts.
It is definitely an issue that dealers and collectors will want to look at closely when purchasing.
Likely started as 1909-D
The altered example shown here most likely began as a genuine 1909-D Indian Head half eagle from the Denver Mint.
Using precision engraving tools and quite a bit of skill, an alteration expert reworked the D Mint mark into a fake O Mint mark.
As you can see from the photomicrograph, the resulting O is fairly well done, and appears to blend into the surrounding fields better than most alterations.
Compare to genuine
When compared to the O Mint mark on a genuine coin, the alteration becomes more obvious.
The genuine Mint mark is more oval than round, and has a narrow vertical inside opening.