There was a similar post about a week ago that had an american flag in a similar position on the starboard side. WD explained it as the flag flying in the prevailing wind position as if it were on a flagpole, front to rear. I see a similar case in this scenario with Sabena. Similarly, in the U.S., if you hang a flag vertically as opposed to the normal horizontal, the stars must remain in the upper left which in certain cases means, flags being displayed on their ‘bottom side’ which at times displays stitches as opposed to the top.

P.S. I love that Olympic Comet in the background of that Sabena prop-job in LHR circa 1963.