That's an German MP-40. Don't think it's a one-off, given that the Nazis made over a million of them before they got out of the business in 1945.
+1, and it's worth mentioning that the Soviets captured many thousands of these on the Western Front, but IIRC didn't deploy many of them during WWII because they could only have been used with captured German ammo. However, the Soviets were famously loath to throw any
weapon away, so the MP40s were probably warehoused just like all the other captured Nazi weapons.
I surmise that the gun was probably given to Mao Zedong's forces while the Chinese and the Soviets were still on speaking terms.
During this period, the Chinese were so desperate for firearms that they would accept virtually anything from anybody, and their forces were armed with a bewildering variety of hand-me-downs from all over the world. However, once things started to stabilize in the 1950s, the Chinese started standardizing to solve the logistical problems caused by having so many different weapons and cartridges.
The Chinese may have given this MP40 to Ho Chi Minh's forces with the idea that they could utilize captured French ammunition.