i'd guess it is probably an MPB040 ABN or ACN?
you can find the serial number on the frame of the truck usualy inside the motor compartment around the base near the right side, on some i think they stamped it in the frame plate on the front in front of the battery, and others i've seen it stamped across the top frame behind the battery along the top where the dash plate is.
Yale had to different serial number types, earlier models begain with an Nxxxxxx (followed by 6 numbers)
later on they went to the 11 digit system like B807NxxxxxA (something that resembles this)

There are 3 places to check :o)
if you find a serial that will help alot