BH wrote:
Looking at these pix of the Balboa, it is clear that this was going to be a non-removable roof.

However, it looks like the opening for the backlight is a channel rather than a flange. From the shot looking in through the interior, the angle of the channel relative to the wheelhouse, suggests that that they might very well have been able to make the glass go down (rather than be fixed). The only remaining question would be one of structural integrity with such a feature - might have resulted in a lot of shaking over rough roads.