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In Stafford's The Big E, he states Mehle, Wade McCluskey, and Jim Daniels attacked and shot down a twin-engined bomber that was attacking Enterprise, but doesn't say whether Mehle was given a whole (or partial) credit for it. He also states that Mehle later attacked and shot down a twin-float seaplane.

In Lundstrom's The First Team, he states that Enterprise was attacked by 5 G3M twin-engined bombers ("Nell") the afternoon of 1 Feb 42, which were intercepted by several VF-6 pilots. Mehle was not one of the two pilots who managed to rough up the lead Japanese bomber, which later crashed after striking the deck of the Enterprise. Lundstrom states that the remaining 4 bombers returned to their base, which would seem to refute the claim for the shoot-down (and Mehle's part in it) described in The Big E. Lundstrom does go on to state that Mehle shot down an Aichi E13A1 twin-float reconnaissance floatplane ("Jake") later that afternoon, which is consistent with Stafford's account.

So, the answer to your question appears to be: Aichi E13A1 "Jake" floatplane.