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Knuckles in Sonic 2 prototype 0606 for the Sega Mega Drive was released by drx during the February 23rd, 2008 proto release.
This prototype, along with every available build after the 0524 prototype, is a dump of the Sonic & Knuckles UPMEM chip. In order for this build to be playable, it must be attached to the beginning of a Sonic 2 ROM. Merging these two ROMs emulates the use of Lock-On Technology. The Knuckles in Sonic 2 0524 prototype is the only prototype released as a working stand-alone build.
- Level Select is not enabled by default. At the title screen, pressing , , , , , and then + Start will enable it.
- This is the first version to use the "Knuckles the Echidna" banner on the title screen.