If this shotgun was made in Europe (which is likely for an underlever), there should be a series of proofmarks on the bottoms of the chambers; the smaller secondary underlever should spin in the opposite direction to let you take the barrels off, and if you can take some pictures of any of those marks, you'll probably get some more information.
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Gun control in Canada: making the streets safer for rapists, muggers, and other violent criminals since 1936.
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