Haenel Model-28R - In detail.
Here is an explanation of how this fixed-barrel ball repeater works (with thanks for input from John Griffiths, author of The Encyclopedia of Spring Air Pistols):
There is a spring-loaded gate that you can see drops down from the cylinder just forward of the trigger/guard when the action is opened.
A lead ball then moves forward from a fixed magazine tube that sits in the centre of the cylinder, under pressure from a spring loaded plunger that slides inside the removeable magazine tube (missing from the example below).
When the gate returns to its closed position after the mainspring is fully cocked, the ball is lifted to the correct position in the breech ready for firing, and the rest of the magazine is sealed off.
To load the magazine, you first feed the ball shot into the fixed tube. You can then screw the magazine tube into the threaded end of the inner tube, so fixing it in place with the ammo under spring pressure.
With thanks to Frank for these pics.
This pistol is currently for sale on eGun.de by a British seller.
Edit: this pistol sold for 2010 Euros hammer!
Haenel Model-28R (rusty)
It looks like the small torsion spring inside the grip frame that forces the cocking link to engage with the piston has broken or become detached - a common problem with this pistol design (see here for details).