Which traffic sign refers to the permissible total mass?

You answered correctly with sign 2. This sign, which shows a truck symbol together with a weight (7.5 t), refers specifically to the permissible total mass of vehicles, typically for trucks. It means vehicles with a total mass above the indicated limit are prohibited.

Sign 1 only indicates a general weight limit for all vehicles, not specific to permissible total mass for certain classes like trucks. In the German rules, the sign with the truck symbol and a tonnage plate directly concerns the permissible total mass. Always pay attention to vehicle symbols and additional text on signs to understand exactly which rule is being applied.