Hiroshige was a member of the Shitahara school of smiths in Musashi province, but he produced this sword in Ise province, the home of the notorious smith Muramasa. Muramasa was known for the distinctive shape he used for the tangs on his swords, the so-called tanago-bara ("bitterling fish belly"). The tang on this Hiroshige sword is also a tanago-bara shape.
For another Hiroshige katana bearing a nearly identical signature and date in the collection of the Kuwana shrine in Mie prefecture, see Murakami Kosuke, ed., Toko Shitahara kaji (Tokyo: Tokyo to Kyoiku Iinkai, 1969), p. 182.
For a ken with the same signature and similar date in the collection of the Kasuga shrine in Nara prefecture, see Matsumoto Masatoshi, ed., vol. 61 of Token Bijutsu, p. 12.