Because it's clear & not tinted, it would suggest that it was meant to be looked out of by human eye. You did not describe it as having any distortions when looking through it. My guess therefore based upon your initial observation & photos along with you describing it (possibly from WW II era) would be a Perspex of some kind for say a navigator to take sextant readings. A B-24 (& Lancaster) bomber had such a clear glassed dome of the approximate size immediately in front of the pilot's windshield on the top deck, which incidentally is where the navigator's station is located below. As near as I can recall, I believe it's mounted in such a way that the opening is slightly smaller than the flange at the base of the glassed dome, thereby "wedging " it within the fuselage. This is all pure conjecture but you asked, & I'm giving it my best guess.