[lugerholsterrepair]

Diego..I have blown up your photo and you are NOT missing the holdopen spring. I can see it is there but it is wedged under the pin that holds your holdopen in. Lift the L shaped portion at the rear..GENTLY! And pull back, your holdopen will come out. You may need to reshape the spring as it looks pretty bent.

When you go to re install the holdopen..the flat spring slides over the top of this pivot pin and hooks the holdopen underneath..the spring, the cross pin and the hook act in concert to hold it all in place.

Your flat holdopen spring must have a rainbow arc ..the opposite of what it will be when you get it out. This gentle arc of the spring is what holds the L shaped portion of the holdope into it's well..escaping the movement of the bolt which rides/slides over it until the last cartridge is chambered. Once this cartridge is in the chamber the follower button pushed by the magazine spring is then applying pressure to push up on the holdopen, overcoming this small flat spring and as the bolt returns on it's rearward travel the holdopen slides under it with upward pressure..until
the bolt returns forward. At the point it begins it's forward motion the bolt is caught by the knife edge of the holdopeon on a notch cut underneath the bolt.

From what I can see in your photo's your holdopen was simply installed incorrectly. You MAY be able to salvage it.