Those plastic covers were designed to do two things: Shift the headlight beam five degrees to the right, and tint it yellow. They were meant for temporary installation on English cars that had been taken across the English Channel to France, where yellow headlamps were required from 1936 until 1993. The rightward shift was because they drive on the left in England, so English headlamps produce a low beam focused downward-leftward (away from oncoming traffic in a left-traffic country) instead of downward-rightward like the headlamps in France or any other right-traffic country. The covers did a minimally adequate job for temporary use in France, but that's it. It's not safe to drive with those covers in place over RH-traffic American headlamps; your seeing distance won't be anywhere near adequate.
(And no, they're not made any more; not only does France no longer require yellow light, but England no longer uses round sealed beams)
