First off to equate "Guild Gun" and this example with somewhat of a swan's neck and long trigger-guard bow would border on heresy. There wasn't such an animal and the term has been attached to examples that are devoid of the name of a firearms merchant, such pieces originating from a cottage type industry. So the term trade-gun is more inline with most examples. Nah, Simson didn't make this. They were probably focusing on military contracts, autos and bicycles about this time.
The engraving is unique and I can't put my finger on it but maybe Axel E. can attribute it as he is an authority on the craftsmen of Zella Sankt Blasii & Mehlis. It has a side frame reinforced receiver and lower scears. First the monthly ledger number just below the proof date gives it away as does the script Nitro stamp(post Sept. 1st, 1911 in Z-M) on each side of the tubes. So, the twin tubeset longarm was made by a Zella-Mehlis craftsmen. Then there's the "K". Single letters are very difficult to assign or attribute and most have sworn off attaching initials or marks to individual mechanics as there just isn't much recorded information and the effort is purely speculative. But Karl, Kehl, Kirchner, Klett, Kommer, K÷nig are just a few of the last names of the laundry list of potential choices. But I would guess the single "K" to note an effort by a Z-M barrel knitter and 1st to mind would be Edmund Keller. He may have also performed the final effort, inside & out, on the tubeset. But the "S" in the Chevron is the mark of a Schilling forge and directs compensation, and liability, their way. I'll look for similar Z-M boxlocks,and Axel E. might know right off, but my 1st guess for an all inclusive facility, would be that it could have originated at the firm Heinrich Christoph Klett & Sohne. With them, all tasks could have been performed inhouse as they were into tubes and could also account for the single "K". Any marks on the standing breech? Somewhere there are a few clues that will give up the craftsman. Any additional marks on the other set of tubes?