IC 3366 is a faint galaxy just south of NGC 4424. Schwassmann made some error in his measurement or reduction of it and a nearby star -- both are just 4.0 seconds west and 15 arcsec south of his positions. His description of the galaxy is appropriate: "vF, vS, nf surr m n". The last bit "translates" as something like "surrounded by much nebulosity north-following." (He uses the same phrase to describe IC 3339 = NGC 4411, which see, where the description is a bit more apt because of the star superposed on the galaxy.)
Whatever happened, I am confident of the identification because of the note about the "nebulosity" to the northeast. This is clearly a reference to NGC 4424.