You're gonna have to take out the bulb anyhow to replace it, so why not take it out now and bring it to Wal-Mart to compare?
Anyway, the bulb is not limited to MB's so you won't find a MB part # on it, if that was what you were looking for.
I'll go see if I still have the spare in my glovebox.
*edit -- saved only the top portion of packaging but here are some pics. It is literally the smallest bulb they sell I think.