This is a 1813 Capped Bust Half in PCGS AU55.  It is the 50C/UNI, O-101a.  From the PCGS Coin Facts website:
 
"One of the most unusual engraving errors on 1813 Half Dollars is the 50C over UNI variety. The normal procedure for engraving a die in 1813 began with a hubbing of the eagle and the scroll into a steel die. Following that, the engraver began placing the legend around the central design and the denomination beneath the eagle. Clearly, the engraver of this die was a little confused when he began adding the legend to the back of the coin. Instead of beginning the word UNITED in the normal position above the tip of the eagle's right wing (viewer's left), the engraver began punching letters adjacent to the lowest arrow. He added three letters (UNI) before realizing his mistake. Rather than discard an expensive steel die, the engraver completed the die as normal, placing the denomination over the misplaced UNI. This was not unprecedented, as there are numerous overdates from this era, as well as inappropriate star counts, and misplaced letters.  Relative to the overall population of 1813 Half Dollars, the 50C/UNI is about ten times as rare as the "normal" varieties."
 
This example exudes flashy luster under the light.  Mostly a light patina with steely surfaces that have just a touch of colorful toning on each side along the rims when viewed with the right angle of light.  No major detractions, a very PQ high AU Bust Half.  
 
This coin is approved by CAC.
 
PCGS Cert. #19384005