Ooooh...worse than I thought. The kit is a straight Vietnam-era M109; at least is is supposed to be.
The problem is that the kit doesn't have the full mantlet, it is made for the cover to go over it and hide the top. The kit cover does not even look like the one in the picture you added, it looks something like a Venetian blind, which was probably a West German modification.
I was hoping that American vehicles would have used the same cover as on the later M109s, as you show in the picture, but apparently not.
Which leaves me with two options: either scratchbuild the mantlet, which would be difficult as the turret is to a great degree already assembled, and I don't like scratchbuilding anyway, or use putty to fill in the grooves on the kit part in order to make it look more like the American cover, and hope that the people who look at the model know even less about the M109 than I do!
Thanks!