British intelligence saw Yamato but misjudged her gun caliber

How did the British naval attache in Japan miss the Yamato class?

He did not miss it entirely; Britain knew of the ships but, with limited yard access, estimated their guns at about 17 inches.

The naval attache in Tokyo could not freely inspect Japanese shipyards, so physical access imposed a hard intelligence limit. British observers nevertheless gathered information through attaches and a broader espionage network. Their estimate placed Yamato's main battery at roughly 17 inches rather than the actual 18.1 inches. The case illustrates the difference between detecting a secret program and resolving its technical details accurately.