Almost all of the coachbuilders like Zagato, Allemano, Nardi, Viotti, etc. marked the body panels, upholstered panels, seats & alloy/metal trim pieces like this. You will often see the car's particular coachbuilder ID number (not the chassis number!) stamped onto the back of each metal trim piece (front grille, exterior trim mouldings, window mouldings, window-winder mechanisms, door latch mechanisms, etc.), as well as hand-written on the backside of the interior upholstery panels or stamped into the seat frames. The writing on the body panels is still sometimes visible inside the door shells or underneath a front/rear decklid. These numbers will normally all match (all the same number), unless an item has been replaced.
All of the Abarth 750 Zagato & Abarth 750 Allemano cars I have owned or have worked on have had coachbuilder identification numbers stamped/written in this manner. My 1956 Fiat 600 Viotti Coupé also has numbering like this written on the back of each upholstery panel, as well as being stamped on the seat back frames and under the front hood near the latch.
My car has the number "514", like this:
