I'll have to have a look at my 1870, my interest is piqued by there being rare examples! I wouldn't agree it is common - I think no bun date after 1863 and before 1874 is an easy date, especially in higher grades. It's commoner than 1868 certainly, but scarcer than 1872, 1873, 1866 I would say. Probably on a par with 1865, perhaps scarcer?