Best bet on steel proof rating is to take it in to a gun shop/gun smith if you can. I spent hours researching mine (the week before opening weekend), and ended up taking it in to Gunworks, and they checked with the rep. I had a look at Miroku guns proofing, and couldn't find anything too helpful for you, other than to check the symbols under the barrell... couldn't find much on what they mean though. Sounds like it fits well, so might be worth the effort.
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