Some episode IV-VII reference shots:
ANH
The gun as based on a modified cast of a Sterling SMG. Notice it's a tad too small for a full-sized Sterling, and the barrel is slightly bent. The magazine housing is missing, the scope is an original Single Point with clear fore-end and red rod:
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ESB
This is clearly a cast too. This time the scope is cast with the gun as it has a black fore end. The gun is however stretched relative to the ANH version. A section of the receiver in front of the ejector port is preserved. The scope rail has been expanded to a rod and now stretches out over the aluminium fore.end. The lack of holes in the barrel shroud indicate this particular gun may have been modified from an ANH prop:
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This gun is (presumably) a screen used ESB gun. Notice the presence of a magazine well and holes in the barrel shroud:
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ROTJ
This is (presumably) a screen used prop from ROTJ. Notice lit is somewhat shorter than the ESB-version, the section of the barrel shroud with holes is missing, possibly bringing it down to ANH length. It is similar to the ESB version in that it also has a magazine well and short magazine as well as a clearly cast scope. The scope rail again overhang the aluminium fore-end:
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TFA
This picture is from Wired Mag's photo lineup of props. IT's clearly a DH-17, if an upgraded version. The scope has been switched out with a more modern one and moved forward. Much of the scope rail over the receiver is missing, but the forward extension is prolonged. This one too lacks holes in the barrel shroud. I could not find a pic to see if it has the magazine well:
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