I haven't seen this documented anywhere, but there's a THIRD unused hanging sprite (on the very left here). The unused hanging sprites are found in Tidal Tempest (which seems to be a hub for these unused sprites in the final, with leftover mappings as far back as v0.02). In the Wacky Workbench sprite data, all 3 are replaced with the final sprites, even in the 510 prototype, so it's possible that this set is the pre-510 version, before they were redrawn. Also, funny thing about these sprites. On TCRF and Retro's wiki, they were portrayed as a single sprite, colored blue, and a few sprite sheets I've seen color the second frame white. The game data shows that the first frame is black and white, and the second frame is yellow and black.