DSII: Open topped vehicles

I was doing a bit of reading and came across a rather interesting point.
 In
the book it says that open topped vehicles are treated as Armour class 0
for
artillery and SLAMs.  does this then mean that they have a top armour of
0
(i would assume so), and so does that mean that any vehicle must use an
armour value for their top when attacked by arty or SLAMs? (ie 1 less
than
their front armour value)

Just Curious
Michael
Brain:	Pinky are you pondering what I'm pondering?
 
Pinky:	Uh..I think so Brain but, uh..Brain? where did all those
cylinders
graduate from?

Brain: (Whacks Pinky Repeatedly)