I don't know if I can add much of anything to this discussion Researcher; but, at your invitation, I'll add my opinion for whatever it may be worth. First of all, I noted this gun in the auction and came very close to jumping in the bidding (I really liked the nice beads around the breech balls, something not common to SAC guns); I didn't of course, and am glad to know that Reb, a SAC fancier, is now the proud owner. In my SAC research, I found absolutely nothing to indicate the Syracuse Arms Company was working on a single trigger design. This doesn't mean old Frank Hollenbeck or George Horne weren't working on a ST design; but we do know there are no ST patents issued assigned to the Syracuse Arms Company. Robert Chambers sent me some photos of his late Grade O gun with a single trigger (a Jaeger patent) that I believe could have been a SAC single trigger test gun due to the fact that Jaeger (a German inventor) was in the US when this gun was produced. As to the subject gun, the trigger design looks just like a photo of an early SAC gun with a single trigger that Buck Hamlin shared with me some years ago. He didn't think the trigger was factory work; but could not be sure. My personal opinion, given that the stock is a replacement, is that the trigger is aftermarket and the gun restocked as part of the trigger conversion project (although I cannot be certain either, and have no idea who did the work).
As to other questions raised, I have never recorded a SAC "live bird gun". I have not seen another SAC gun with a trigger plate screw as large as seen in this example; I suspect whoever fitted the single trigger wanted a more solid anchor point than the original smaller screw provided. As to engraving, it is not uncommon ot find early examples of A Grade SAC guns with exceptional engraving, and also different engraving patterns (no game scenes, but unusual flourishes of scroll and in odd locations). This is the only A Grade gun I have seen with raised ridges behind the breech balls. Whoever did the fitting and engraving repair did excellent work, the engraving modifications on the top strap and trigger plate appearing to be identical to the patterns on the balance of the frame; but SAC used the Glahn family for their engraving work and those guys could have engraved the modified parts long after SAC was out of business. Again, I don't have the answers but this is a very intriguing gun; I hope its history can be determined, and I'm glad it has found a great home!
