Which two serial numbers must match on the M120 system?

In the M120 mortar system, it is critical for the cannon and breech cap serial numbers to match. This requirement ensures that the components are compatible and maintain the safety and integrity of the weapon system. Each cannon and its corresponding breech cap are specifically designed to work together, and mismatching serial numbers could indicate that the parts are from different systems or batches, potentially leading to operational failures or safety hazards.

The other combinations listed do not have such strict compatibility requirements as those between the cannon and the breech cap. For example, while the bipod and baseplate are important for stability and support, they do not require matching serial numbers to function correctly. Similarly, the barrel and casing or the elevation crank and bipod do not have a direct operational dependency that necessitates matching serial numbers. Overall, the matching of cannon and breech cap serial numbers is essential to ensure the weapon system functions as intended and adheres to safety regulations.