The RM-2 was the last in a series of light machine guns designed by long-time Mexican arm inventor Rafael Mendoza (also responsible for, among other things, the very unusual Fusil Mexico). The first of his machine guns was developed from 1928 to 1933, and ultimately adopted by the Mexican military as the Model B-1933 or C-1934. This was a long-stroke gas piston gun chambered for 7mm Mauser (the standard Mexican military cartridge at the time). It uses a rotating bolt somewhat reminiscent of the Lewis (and in turn, the FG-42 and M60), in which the firing pin is held fixed by the operating rod, and the bolt rotates around it to lock and unlock.
The gun fed from 20-round box magazines in the top of the action, and also featured a quick-detach barrel. All in all, it was a quite competent machine gun, although it was not adopted outside Mexico.
After WWII, Mexico took advantage of lots of cheap surplus arms from the United States and standardized on the .30-06 cartridge in place of the 7mm Mauser. Mendoza redesigned his 1934 machine gun for the .30-06 cartridge, and then went about designing a new model to improve several aspects of the gun and make it cheaper and simpler to produce. This would become the RM-2.
One clever element in the whole line of Mendoza machine guns was the use of a reversible, double-headed firing pin. Should the tip of the firing pin break, a quick field-strip would allow the operator to flip the pin around and be back in firing condition (this can also be seen in the Colt CMG-2 and CMG-3 designs). The design used a simple open-bolt firing mechanism, and had a sliding dust cover to keep the action clean when a magazine was not inserted.
The magazine was mounted on the top of the action, so of course the sights had to be offset to the side (the left side, specifically). A rear aperture was used, adjustable vertically for zeroes from 200m to 1200m. Contrary to the description in Small Arms of the World, the RM-2 did continue have a quick-detachable barrel. It used a horizontal sliding wedge to cam the barrel in and hold it in place.
Thanks to the National Firearms Centre in Leeds, we have a batch of photos of a disassembled RM-2: