Here is an illustration that may help you understand the appearance, function and location of the locking bolt spring.
This great illustration is an extract from the excellent e-book on the Luger Mechanicals by Gerard Henrotin. You can find a link to his e-books on the links and resources page of this website. I highly recommend all of his e-books on Lugers to new Luger owners or enthusiasts.
A- is the tip of the L-shape part of the spring that should protrude through the hole in the bottom of the frame in front of the trigger guard.
B- is the end of the spring that you should be able to see under the round portion of the locking bolt on the right side of the frame when the spring is properly installed.
C- is the indentation on the locking bolt where the bump on the spring (E) rests when the gun is assembled.
D- is the location of the spring when properly installed before the locking bolt has been installed.
The installed spring should provide moderae resistance to movement of the locking bolt with the upper half of the gun removed. If it drops down, then the spring is worn out or the notch in the locking bolt is worn...
I hope this has been helpful.