PeteM:
It was Syndicat Liegeois,
http://www.littlegun.be/arme%20belge/artisans%20identifies%20s/a%20syndicat%20liegeois%20gb.htm , I was considering and trying to remember about what time they began. I think that mark to be a script "M" and to have been stamped from the other side like many initials. "M" is seen on many Sauer examples such as the other set of tubes. "Sl" is also seen on many sets of tubes which I think to have been sourced from Belgium. The lack of marks on the rifled tube is what sets me to pondering. Also, considering how all the marks are worn, at some point and possibly early on, an extra effort was applied to the set. But all the marks are there to indicate that the 2 tube set was completed at Sauer as well as extra effort was expended toward the pattern. Also note the same numbers and letters on the lower rib. Like many, the marks are quite the puzzle.
Kind Regards,
Raimey
rse