Driftmeter in the thirties - anyone?

[Rob de Bie]

I'm doing a detailed drawing of the Pander Postjager from 1933. Details of the lower side of this aircraft are rather sketchy. For example, I cannot figure out the various hatches for control system access. But I do see a window under the fuselage.

I think it is for the driftmeter, but I'm not 100% sure. The radio operator, seated behind the two pilots, had a driftmeter, that is all I know. He was seated on the left, and could lean to the right to view down the driftmeter. What I can deduce from very oblique photos, there's a window of roughly 250 x 200 mm (10 x 8 inch), on the right side of the centerline.

My main question is: what kind of window in the fuselage bottom would a driftmeter from that era require. Does anyone know? Thanks in advance!

Rob