It's actually easy to see if a BEEP is a single motor, one wheelset will be free-wheeling. If it's a dual-motor unit, all the wheels are driven. The reverse board is one factor as Chris states, however the motors are apparently different as well. I've taken a few BEEP chassis and test run them without any electronics, just a bridge rectifier. The older chassis is a lot slower than the newer ones, so that eliminates the reverse board as the primary factor.
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