Why not use mods for the design tests?

Are there mods that could add the missing technology?

Possibly, but this test avoids them so it remains standardized and repeatable with the same game setup.

Mods could provide missing equipment, but they would make the result dependent on a particular modification, generation, and interaction with the base game. The design tests therefore remain standardized and replicable. When the stock technology cannot represent an 18-inch gun in the selected year, the practical workaround is to move the scenario to a later date while leaving the ruleset unchanged.