Unlikely. Plaque 20 g stamped on the watch case, means the metal is not sold gold but plated with 20 microns of gold. You can find Cartier manual wind mechanical movement, watches, stamped "Cartier" "PLAQUE OR G 20 M" "Swiss"
Omega also made (1970s) watches, that were marked on the side 20 (for 20 microns) with the Omega insignia inside a triangle.
Knowing how many microns or how thick the gold layer, will help a jeweler know how to effectively polish your watch with out damaging the surface, or touching the base metal.