Sinner, thanks for your suggestion as well.

Yesterday I tried to examine the rifle again very carefully to see if I might have missed something. Without disassembling any parts I noticed that this rifle has its ejecting mechanism inside the foreend and it seems movable.

So instead of focusing on how to cock the hammer, I learn that there should be a way to move the ejectors in to another position which on the other hand I can say that I had to 'cock' the foreend.

All I have to do is that I grab the foreend and turn the ejectors against the edge of a wooden table and push them down. Then I hear a click!

Yes! it works. I can assemble all 3 pieces together and the rifle now is back in one piece and working again..

-- case closed -- Thank you everyone for reading my story.