Accesorry for HDP 50  - 5 balls storage speedloader  
It is mounted directly on the rail without other screws. 
After you finish firing what you have in the pistol,
you block the supply spring, you turn the pistol and 
the 5 balls from the additional magazine will fall into the main magazine.
New model, you can see inside external magazine how many balls there are left/what type.
Printed 3D.