Generally it's easy with Mercier cause what we mainly ever see are just 3 models: the 300 (all 531DB) the 200 (3-main tubes plain gauge 531) and the 100 (which is what I THINK we see here: a hi-tensile thinner than gaspipe but still plain gauge plain steel) with "fancy" lugs and graphics. The Simplex dropouts are stamped but have some dimension to them...all around sort of what the UO-8 aspires to but Mercier gave you more at around that price-point.
There COULD be other models that were "in-betweeners" in which case I'd say this COULD be something like a "150"...
BTW, that graphic of the rider with hand raised is probably Raymond Poulidor, long time Mercier pro rider and very popular in France.