Here are some of the comments I got from the sites I was browsing while looking for reference pictures for the model:

"The actual weapon is a modified German MG34 machine gun.  (You can see this gun in Saving Private Ryan, in use by the German troops)  Most of the added "acessories" are actually bicycle parts, including grips, a brake lever, and front suspension.  The weapon is stabilized using a steady-cam harness.  Steady-cams are used by filmakers to keep the camera from moving while the camera operator walks along."

and from another site:

"The prop was created using a modified 2nd World War German MG-42 machine gun. The “trigger” is a Kawasaki GT motorcycle brake lever, the front barrel dressing is derived from a motorcycle foot rest and the grips are Kawasaki GT switch gears. The muzzle brake (the very tip of the gun) was specially made to alter the look of the barrel and adds to the overall detail and to create a unique looking weapon."

And the image is supposedly the official technical drawing of the movie version.

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