Historically, some arcade hardware and early emulators used key files for copy protection or to enable specific game features. A reference from 2008 mentions looking for "xors" (XOR keys) for DDTOD and other games, which were likely used for decryption or to bypass security measures. An "exclusive" ddtod.key could refer to one of these rare, community-developed patches or a key from a specific, hard-to-find regional version of the game.
Look for "super-users" who consume content or products at 5x the average rate. ddtodkey exclusive