I agree with the upthread comments: this is a die crack. I believe that it is a keeper, however. (Hey, it only costs one cent to keep it!)
Die cracks between mint marks and curved numbers in dates are common during the high mintage and die overuse era during and after WW II, with several of them developing into known interior die breaks (IDBs) that are listed on cuds-on-coins. Date-mm IDBs are known for 1950-D, 1951-S, 1952-D, 1952-S, 1954-S, 1957-D, 1979-D, and date-only IDBs for 1983 and 1987. There is presently no known IDB for 1973-D, but your coin could be a break that later developed into one. It might be fun to keep it in case an IDB is discovered, since you would have an earlier stage. The Denver mint overworked many dies in the 1970s, and anything is possible with overworked dies.