From the Ameliapedia
"The Lockheed Electra 10E was fitted with low distortion windows in several positions to minimize refraction errors during celestial observations. [16] (Military navigators were later cautioned to avoid using windows for celestial observations, and not to observe celestial bodies below 11 degrees, due to the excessive refraction error.)[17] [10]"
http://tighar.org/wiki/Air_Navigation:_State_of_the_Art_in_1937 The citation is [16]Wright, Monte Duane, Most Probable Position, The University Press of Kansas, Lawrence/ Manhattan/ Wichita, 1972, p. 156
Since this window was probably installed at the request of Manning, it seems likely he would have asked for a flat window to minimize distortion/refraction. Would there have been a viable case for him to want a window that opened to get totally distortion free measurements or would the wind/air speed made that impractical.