these can be found on the lugerforum website. on the left side in the black column.
http://www.lugerforum.com/
under technical information. them are proof marks they are the first letter of names of inspectors under crowns.
Proof Marks : Proof 1 Proof 2 Proof 3 Proof 4 Proof 5 Proof 6
13. German military receiver proof. Found on 1908 military examples with chamber dates.
14. German military proof. Found on barrel and breechblock, 1908-1913, and breechblock only, 1914-1918.
15. German military receiver proof. Found on DWM Lugers 1914-1918.
16. German military receiver proof. Found on Erfurt Lugers produced 1908-1918. Minor variants noted.
17. German military barrel proof. Found on DWM and Erfurt Lugers produced 1914-1918.
18. Inspector's proof found on small parts of Erfurt produced Lugers. One or more may appear on the weapon.
19. Erfurt and DWM arsenal proof. Indicates arsenal repair, refurbish, or modification.
20. Arsenal proof found on 1908 military Lugers to which the holdopen device was added.
21. Barrel proof indicating the barrel is made of Bohler steel. The digits refer to the grade of hardness.
22. DWM barrel proof. May be found alone, or with commercial or contract proofs.
23. Dutch proof. Found on the 1906 contracts (DWM and Vickers). Referred to as a Queen Wilhelmina proof.
24. British proofs found on Vickers manufactured Lugers.
Courtesy of Ted