When the Allies identified Yamato's 18-inch guns

When did Allied intelligence determine that Yamato carried 18-inch guns?

The answer places the assessment around late 1942 or early 1943, before the war ended and after Far Eastern intelligence work narrowed the uncertainty.

Prewar intelligence debated whether Yamato carried 16-, 17-, or 18-inch guns. Far Eastern reporting gradually narrowed that uncertainty after the war began. By late 1942 or early 1943, Allied analysts were described as fairly confident that the main battery was 18-inch. A concealed design feature had become a concrete operational concern well before the war ended.