King, you're confusing a couple things. The Miroku 500, most often seen under the Charles Daly name, is a sxs, not an OU. Their OU models had names, not numbers.

Mitsui didn't actually "make" anything. After Ithaca ceased importing the SKB doubles (both sxs and OU) in the mid-70's, they were brought into the States (and possibly Canada) for a very brief time under the Mitsui name. Mitsui, I think, was just a marketing entity, not a gunmaker. Differences between a Mitsui 500 and an Ithaca SKB 500 were minor, about the only one you'd notice being that the Mitsui-imported guns didn't have the Ithaca "Raybar" front sight.