Genesis:Expansion Cover Information
Most original releases of the 32X came bundled with a part officially called the Extension Unit, which plugs into a Genesis when it is not attached to a Sega CD. This was mainly intended for the VA0 model 2 Genesis, as the 32X exhibits unstable behavior on this console when the expansion port is not in use. Inside the Extension Unit are a set of printed resistors which act as pull-down resistors (meaning connected to ground at one end) to all 16 data bits on the system bus. The Extension Unit was not mentioned in the 32X manual and Sega printed a supplement to go with it, instructing users to connect the Extension Unit if they were not using a Sega CD, regardless of which Genesis console they had.
The later VA1 revision of the 32X (model number MK-84000A) has these pull-down resistors built-in, making the Extension Unit redundant on these 32Xs. Accordingly, Sega did not bundle the Extension Unit with VA1 32Xs.