The PM9 was an interesting an unique submachine gun designed by Louis Debuit for the French firm Merlin and Gerin (hence the MGD name – Merlin, Gerin, Debuit) in the late 1940s and early 50s. The design was intended to provide a very compact package, which it did with a very short action, folding stock, and folding magazine.
The PM9 uses a delayed blowback action, and the delaying is done by a rotating flywheel-type block and clock spring. The bolt and flywheel act somewhat like the piston and crank in an engine. As the bolt (piston) moved rearward in a straight line, it forces the flywheel (crank) to rotate because the two are connected. In the case of the PM9, the connection is a nub on the flywheel that rides in a vertical slot in the bolt. The flywheel is pushing against the clock spring to rotate, and the combination of the its inertia and spring pressure keep the bolt closed long enough for pressure to drop to a safe level. The rotary action allows this to be done in a much smaller package than typical submachine guns.
The PM9 was initially chambered for 7.65 French Long, but quickly changed to standard 9mm Parabellum for the production models. It used the same magazine as the German MP38/MP40, giving it a 32-round capacity. In addition to the model with a skeletonized folding stock, the PM9 was also available with a fixed wooden stock and either short barrel of long carbine barrel. A relatively small number of guns were produced in France in 1954 and 1955, but they failed to find commercial success. In 1956 the German Erma company acquired a license to build the PM9, but abandoned the idea after making a few prototypes.
Article (in German) from Waffen Revue magazine on the PM9 (with photos and exploded view)
US Patent #2,843,023 (L. Debuit, “Recoil Mechanism for Recoil-Operated Firearms”, July 15, 1958)