I have one which was changed from a standard single trigger to DST.  Its tough to tell but if you look closely you should be able to see where the new slot was cut for the rear trigger.  I believe Campbell, I cannot remember which volume, says this was done at the factory and often there are initials stamped into the tang by the gunsmith who did the work.   
Of course, gunsmiths like HM Pope or Schoyen etc... could have done this as well.  Mine are so well done that it looks an original and I believe the parts are original Winchester parts.  There are letters stamped into the tang but they are not legible to me.  There is also an assembly number on the tang.
Maybe some of our more knowledgable and experienced Winchester collectors can add more info than I can.
Red

Podufa, about 20 years ago a Rifle showed up at our club. It was a Schuetzen model with Helm stock and Bicycle Handle Palm Rest. The Serial number was the early Scroll type of stamp. The Double Set Triggers were the wide spaced version. But! heres the Hook.  The Set Triggers were a in an inserted block in the lower tang. Now they appeared to be Factory work as the Case hardened finish although worn was in good appearance and the colors although faded were continous over the lower tang and the inserted block. The block contained everything needed, both triggers, the adjusting screw, and the kicker spring for the rear trigger and the set trigger spring just as the reguler set trigger tangs had. Functioned good. The rifle was in 32-40 caliber and had a #4 full octagon barrel with the top flat Matted. A truly beautiful rifle and shot as good as it looked. A number of us examined it closely and the owner did remove the buttstock so we could examine the inside of the lower tang. It was stamped on the side with an assembly number that was also stamped in the lower tang slot in the stock. The set trigger block was held in place with two dowel pins that went right across the lower tang and the finish on the end appeared to go along with the rest of the lower tang. We all felt that this was all factory and wondered if it was one of Winchesters early efforts at set triggers. HTH, Regards, FITZ

I suspect mine was done before proof marks, also I think mine was rebarreled , the writing on the barrel does not have the dashes at the start and end . The upper right tang has some marks , hard to see old age and cataracts , I think 2       17.