In 1873, the Mint raised the weight of the Dimes, Quarter Dollars and Half Dollars ever so slightly. Just as in 1853-1855, arrowheads on either side of the date indicated this change. For the Quarter Dollar, the arrowheads were used only in 1873 and 1874, then removed in subsequent years. The arrowheads in 1873 were more cosmetic than anything else; in 1853, the lack of arrowheads indicated that a silver coin contained more bullion value than its face value, thus it could be pulled from circulation and melted down for a profit. Such was not the case in 1873, because the new coins with arrowheads were worth more than the 1853-1873 coins, not less.
Apparently, this design change sparked a collecting frenzy, as the number of surviving Mint State examples is the highest since 1862. Most Mint State examples fall into the MS-62 to MS-64 range. Gems are scarce and superb exxamples are downright rare. The best example is a single PCGS MS67.