That appears to be a very early production gun with the mono-bloc barrels and the "Browning Non-Crossfire Ventilated Rib". However, the single trigger looks like it is a much later single trigger. Certainly not either of the single triggers offered when that gun was new. Methinks things have happened to that gun since it was new. Whether they happened at FN, StLouis, or somewhere else, I can't hazzard a guess from one internet picture. The engraving does look good to my eye and some of the elements of it smack of Rudolf Kornbrath to me.
