I agree with Canvasback. A 1668 (second series) Lindner Daly would put the gun in the 1909 production range, plus or minus a year.
The first Lindner Dalys with 1912 German proofs show up at approximately the 2000 serial number mark (second series). So with a production of 100-150 guns a year, a 1668 serial number puts you circa-1909.
One follow-up note the "about serial number 2000" marker for post-1912 guns: there wasn't a hard-cutover. What I mean is that I have observed Lindner Dalys with 1912 proof marks with a slightly lower serial number than Daly guns with only the 1892 proofs. The serial number differences weren't large and are likely explained with the variances in production times of different Daly models and quality levels.
Ken
