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Welcome to Retro Forum!  Could you post a pic? Block out a couple digits if you must (though I have no idea why anyone would care about anyone else's number), but a nice big pic of the entire side of the lower would be helpful... It's possible it signifies some modification...or perhaps resale of a seized or LE weapon... (woo...ranchhand gots quick fingers) 
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I think the x is from when Colt did replacements on SP1 receivers. If you had a wore out receiver, you could send it to them and they would replace it with same numbered receiver only with an X added. I had a friend who did this, don't remember the cost, and seems like I remembering him saying that it had the X added. Nambu JMHO |
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