The Stg 45 Kurz SMG is an extremely rare and obscure piece of German weapons history, emerging in the final months of World War II as part of the Third Reich’s final efforts to create cheaper, simpler, and more efficient weaponry.
In 1944–1945, Germany, in economic and military collapse, sought low-cost, highly efficient solutions to replace the expensive and complex StG 44.
Mauser developed the Gerät 06 and 06H, evolving into the StG 45(M), which used a roller-delayed recoil system—unheard of at the time, and revolutionary.
Origin of the StG 45 Kurz SMG
During the development of the StG 45(M), Mauser reportedly experimented with a compact and lightweight version, aimed at:
Second-line troops
Armored vehicle crews
Paratroopers
This version, never officially named, is now known by historians as the “StG 45 Kurz SMG” (a modern descriptive, unofficial name)
Technical characteristics (reconstructed by experts):
Caliber: 7.92×33mm Kurz (same as the StG 44)
System: Roller-delayed recoil
Fire mode: Semi-automatic and automatic
Rate of fire: 450–600 rounds/min (estimated)
Feeding: 20-round magazines probably
Length: Shorter than the conventional StG 45
Produced only in prototype form, in very limited numbers (possibly 1 or 2 units).

Looking this image it appears to have 20 rounds
If this weapon existed I think it would be a good weapon in an event or premium paratrooper squad.