According to the book, 18287, and 18288 were the same configuration, so it's entirely possible that the barrel and fore end got put on the wrong gun. The actual SN is that which is stamped on the receiver (as opposed to any other misc. part) so unless you were lucky enough to find 18288, you'll never know for certain. If you had my luck, you'd find 18288, and find it had barrels stamped 18289.
I just thought of another scenario. Both guns were ordered by the same hardware store, and got mixed up when they arrived.
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