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An MBU with the straight 'flush' fit are MBU-100's that were upgraded to the V-3 spec. If they have the 'bow', they were made originally as V-3's (MBU-103). When first starting them, you can hold in the 'start' button for up to 30 seconds at a time, no more than four times before pulling the unit out and replacing it. HTH.![2 Cents 2cents 2cents](https://www.steelsoldiers.com/images/smilies/icon_smile_2cents.gif)
![2 Cents 2cents 2cents](https://www.steelsoldiers.com/images/smilies/icon_smile_2cents.gif)