That is correct. All of the dies were created and mintmarks, if needed, were applied at the Philadelphia mint until the late 1990's when the Denver mint opened thier own die shop and began making their own dies. (Philadelphia still provides them with the master hubs.) There are examples of correspondence from the branch mints back in the 19th century informing Philadelphia that some dies had arrived lacking the mintmark and that they were being returned. Please send replacement dies with the mintmark.