I think it is more likely that this was a damaged upper receiver that was salvaged by a resourceful gunsmith tinkerer  (no pun related to Ed)... by machining off the face of the upper and using a homemade barrel... This is not a factory modification in my opinion.
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regards, -John S
"...We hold these truths to be self-evident that ALL men are created EQUAL and are endowed by their Creator with certain UNALIENABLE rights, and among these are life, LIBERTY, and the pursuit of happiness..."
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