Quote: Can you assume it was 1906 because of the receiver date?
Of course "1906" dates the rifle! you have to understand the German military numbering system: 1) model of the gun, here "Gew98". 2) factory, here "Waffenfabrik Mauser, Oberndorf". 3) the year of manufacture, "1906". 4) a serial number for that year, 1906 in this case. Each factory started each new year with "1" and continued to "9999", only four digits. After 9999 the numbers restarted with "1a" and so on until a new year started with "1" again.
The sear proper is of course numbered with the rifle as it was not replaced when the set trigger was installed. Only the military trigger itself was replaced by the small kickoff lever. The set trigger assembly may indeed have been installed by the Mauser factory commercial shop on your rifle on special order. Nearly everything was possible then at the Mauser factory. At least neither the AKAH and Stukenbrok catalogs don't mention a set trigger option on these rifles. As I wrote, everyone was capable of installing such an aftermarket set trigger. But this set trigger assembly is not of the Mauser design. Compare the spring arrangement to that on your DWM sporter. The double set trigger on your Mauser Armeemodell is definitely of the Suhl design. Maybe Mauser bought it in from the guntrade there because it was cheaper and easier to install into the military type bottom metal instead of replacing all bottom metal with a Mauser commercial one that was specially machined to take the set trigger parts. On those basic "Armeemodell", later "C type" sporters a main objective was to hold costs down by using parts from the volume military production.
Please reread Jon Speed's "Mauser-Original Oberndorf Sporting Rifles". All the "Army Model C rifles" he shows and lists are dated 1908 and later, so this may explain the earlier far back sling swivel mounting on your rifle, somewhat impractical for carriing by German stndards.