The buckle has 3 markings on back. First one spells either "DEPOSE" or "DEBOSE". The second marking is "CH. GUYOT" and the third is "63298". Thanks in advance!
Charles Guyot of Paris France invented the particular form of suspenders-strap adjuster buckle you found. He patented several evolutions of it. He first begn making suspenders in 1848, and his brand continued in production for over a century. Your nickel-plated brass buckle dates from approximately the 1880s onward.