Wolff springs has three weights of recoil spring:
http://www.gunsprings.com/index.cfm?...s&cID=1&mID=38
I believe that the one you have is the 36lb spring.
IIRC, these would be appropriate for 7.65mm Luger pistols and early 9mm guns.
Sturgess, on page 905, discusses spring versions. He says early P.04 and P.08 guns had 18 turns of 1.5mm wire. 1916-1917 dated pistols had a new standard P.08 mainspring 22 turns of 1.4mm wire. There were also changes to the mainspring well.
On page 1093 there is extensive discussion.
The LP08 started out prototyped with 22 turns of 1.4mm wire, was produced in 1914 with 18 turns of 1.4mm wire, and then in 1916 with 22 turns of 1.4mm wire.
The P08 in 1913 had 18 turns of 1.5mm wire and then in 1915 had 22 turns of 1,4mm wire. The frame wells were shaped differently near the bottom of the grips.
The changes relate mainly to the mass of the receiver and the power of the cartridge. It all relates to the balance of the action's operation.
On page 1061 he lists specifications:
3.2 Recoil Spring
Luger 1899 (double) Spring steel, spring hard, finished as sample (Old Model)
DWM 1904 19 turns. Load at a length of 50 mm: 7 to 8 kg. Load at a length of 34 mm: 15 to 16.5 kg.
I.K.B. 1913 Cast crucible steel wire, spring hard, or naturally hard crucible steel wire. 22 turns, both ends bent even.
Mauser 1934 Cast steel spring wire “Extra-Extra”, carbon content 0.8 %. Anneal to yellow colour.
Mauser 1942 Spring steel, carbon content 0.8 %. Anneal to yellow colour.