Assigning a date to a Voirin bow is, to some extent, guesswork but very educated guesswork. One clue is the placement of Voirin’s stamp, as the location changed between 1870 and 1885, the years he worked on his own. Initially the stamp was on the audience side, away from the player, very near the grip, as in this bow:
This is also where bows from the Vuillaume shop were typically stamped
ISAAC SALCHOW FOR SALCHOW AND SONS BOWMAKERS