"How can you tell its the ANH?"
ANH:
The base gun is the E-11 (Sterling L2A3).
The DH-17's in ANH use real Single Point scopes and football shaped scope rings.
ESB:
The ESB DH-17 also uses the E-11 as the base gun.
The Single Point scopes were solid resin castings.
A long rod was used for the scope rail.
The clip and clip receiver were left intact but like on the E-11 the clip was cut down.
The silver barrel was shortened a bit and a small tube was added to the front cone.
Various grieblies were added including the Revel piston halves (as seen on Han and Luke's Mausers) and V8 engine kit parts.
ROTJ:
The ROTJ DH-17 uses the Sterling MGC as the base gun with greeblies added.
The scope is a casting of the Single Point.
Like the ESB blaster they left the clip housing intact and used a shortened clip like on the E-11.
The scope rings are the round Weaver type rings.
For more check my DH17 thread on ASAP:
http://pub23.ezboard.com/fpropreplicasfrm3.showMessage?topicID=233.topic