Prototypes
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Six manufacturers were solicited for prototype designs: Gustloff-Werke, Mauser, Johannes Großfuß Metall- und Lackierwarenfabrik, C.G. Hänel, Rheinmetall-Borsig and Heinrich Krieghoff Waffenfabrik.[8][12] Several contracts were awarded but only a few prototypes are known to have been submitted. Mauser offered a version of the MG 81 (rejected due to excessive weight and its belt-fed operation)[14] while Krieghoff presented a rising-block prototype, which too was quickly dropped. A design credited to Rheinmetall-Borsig’s Louis Stange of Sömmerda who had previously worked on the MG 34 proved satisfactory and underwent military trials conducted by the GL/C E-6 test station at Tarnewitz in mid-1942.[8] This early prototype, known under the factory designation Gerät 450 (“device 450”) or Ausführung “A” (“type A”), was intended to be a pure sheet metal design, using pressed steel in the construction of the receiver, buttstock and corrugated handguard. The proposed system of operation was modeled on that used in World War I Lewis light machine gun, with a gas-operated turning bolt action geared to a spiral (clock-type) recoil spring.[15] The type “A” was never produced beyond model form, but the basic design layout was retained for further development.[16]
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Gustloff-Werke, Mauser, Johannes Großfuß Metall- und Lackierwarenfabrik, C.G. Hänel, Rheinmetall-Borsig, Heinrich Krieghoff Waffenfabrik. https://zhuanlan.zhihu.com/p/36077689
We have three Guns of the FG42 including the FG42 1 and 2 and the prototype, one of six possible prototypes. Six companies were solicited to develop the LC-6 criteria or requirements that led to the FG42. Have started looking but it seems there is very little on the other five companies if anyone is better at this than me give it a try.
mausers prototype ? Пулемет "MG 81" (Германия) - характеристики, описание, фото и схемы. 
Rheinmetall FG42 Prototype
Image credit: Senich, Peter, The German Assault Rifle 1939-1945 , Paladin Press, Boulder, Colorado: 1987 Page 240 Historic Firearm of the Month, October 2000. . http://www.cruffler.com/FG42-Prototype.jpg
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post ww2
