Brilliant! Brilliant! Brilliant!
In 1873, the Mint increased the weight of the dime from 2.48 grams to 2.50 grams. To identify the new coins, Mint employees added small arrowheads to either side of the date. This allowed easy identification of the old-style Dimes for potential withdrawal from circulation (although there was little incentive to do so since the face value of the old Dimes exceeded their intrinsic value).
This 1873 With Arrows Dime is fairly common in Mint State, particularly in MS62 to MS64. The best example is a single PCGS MS66+, a coin that once graced the Eugene H. Gardner Collection.
Rare Coins of New Hampshire, sold privately in 7/2007 - Eugene H. Gardner Collection - Heritage 6/2014:30270, $8,812.50