Wanna..Ted is the man on Darnes. Here is the thread on dating early 20th century St. Etienne guns:]
http://www.doublegunshop.com/forums/ubbthreads.php?ubb=showflat&Number=484326You'll see that from 1889 to 1912 St. Etienne measured their chambers in cm (6.5 rather than the usual mm 65). That's a definitive marker.
Here is a Darne SN chart...Ted will caution that there are errors here...
Your SN looks to be "L106." Now this is interesting. Because in the above line (p.4) you'll see two Darnes - C692 with 6.5 chambers and C643 chambered for 70mm - So at the time I speculated that the changeover in St. Etienne from cm to mm chambering for Darnes occurred in the C600 number series. - i.e. mid-1912. From the chart this would seem to make sense. The Letter/number SN series started in 1910 meaning that from 1910 to mid 1912 about 2600 Darne guns would have been numbered - pretty much within the capabilities of Darne to produce at this time. Yours, however, still uses cm but is in the "L" series - clearly a post WWI number....a mystery, conundrum and incongruence to which Ted will be sagely nodding.
Darne by this time were making their own Barrels...Didier-Drevet, a barrel maker of choice for quality St. Etienne guns for years, refused to raise production above 300 barrels a year to protect the quality of their product, leading Darne to consider alternatives around 1900.
