1873 Half Eagles are found with "Closed" and "Open" 3's in the date. The knobs of the Closed 3 are so close that the date can be mistaken for 1878 at a cursory glance. This defect was noticed early in the year and was corrected by opening up the 3 so that no mistake could be made.
In terms of the rarity of certified examples, the Closed and Open 3 are about equal. The Closed 3 is slightly more common in Mint State and the grade distribution is similar for both. Gem examples of the Closed 3 are very rare, with the finest example being a single PCGS MS66.