Wait a minute, the gun is an Express model? Birch or synthetic stocks and a matte black finish?<br>
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If so, it's built on the 28ga action, and will accept any barrel for this action (which is all current barrels).<br>
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A "standard" 20ga 870 is built on the larger 12ga action. These have been out of production for a long time. These serial numbers end in X for 2-3/4" actions and N for 3" actions.<br>
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How about posting the last letter in the serial number?