Jacques— Both the 238 D-Type Jaguar and the 239 Connaught Racing Car, have their model number in exactly that same position, with similar descriptive wording for the car’s accomplishments. I am guessing that possibly Meccano (or the vendor who made the boxes) made a mistake on the model number for the Mercedes, in that area, and did not discover it until many, many boxes were already made. They then had no choice but to black it out. The size of the blacked out area looks to be just right for “No. 237”. Just my guess....perhaps others have a better one?
Best regards, Terry