I've had a Miroku BC My Luck 12 ga. sxs since '98. Mine is a straight stocked "Model M", choked IC and F, with double triggers, probably made in the late '50's and purchased in Japan by someone in the service. After refinishing the wood with oil, I had two minor problems. First the leafspring for the safety snapped; I replaced it with a section of wallclock mainspring--detempered, cut and shaped, then retempered. Second, after several years' use, the toplever V spring broke. After some research, I found a gunsmith in NJ who had a stock of these he'd made up, and sent one. Total cost maybe $50, and I had to file an end a bit. Toplever springs on earlier models like mine had a history of failing, and were eventually replaced, at least on the O/U's, with coil springs. After buying and installing the spring from the gunsmith, I found Wisner's site (http://www.wisnersinc.com/additional_info/Mirokusprings.html), which lists lots of information about Miroku guns, though again mostly for O/U models. While the O/U V-style toplever spring shown on Wisner's site certainly looks like mine, I'm uncertain it would fit. I expect they'd know the answer.
Can't say about shooting steel--I only hunt upland birds. But I've been very happy with my Miroku, something you'll usually hear from owners. It's handsome, light, well balanced, and aside from these two small problems, has been a pleasure to shoot and carry. At under $400, in the condition you describe, clearly a steal.
