*Nope, not a misprint. I did not forget to put "Little" in front of Bighorn.
Reading a book about the 1873 railroad surveys of the Yellowstone valley and find it mildly interesting that Custer with attached cavalry fought a skirmish with the Sioux under Gall and others that he would meet 3 years later. This skirmish was fought at the mouth of the Bighorn River.
Just guessing, but based on several drives through the area this is maybe 25 miles or so from the more famous 1876 events.
Odd, that.