The Stoeger also has the gripframe and 'receiver' [barrel extension] integrated, and the barrel is fixed [pinned, not threaded]/does not recoil.
I would go out on a limb and guess that the integrated gripframe/'receiver' is also of Zinc alloy. Zamak perhaps???...
Ah! Here's a quote from the Wiki -
The first Stoeger Lugers were produced in 1969 with aluminum forging/machined frames (7075 T6 aluminum - tensile strength 82,000 p.s.i ). Beginning in 1980 the receiver material was changed to steel. One way to identify the frame type is by the safety selector markings. Steel frames will have a red and green painted indented spot while the aluminum frames will have raised lettering that says Fire or Safe. Use of a magnet is another way to identify the composition of the pistol frame.
http://en.wikipedia.org/wiki/Stoeger_Luger