Headlights until 1973 were not made in one piece, but had a separate glass, reflector, etc. Later this was improved by using sealed beam headlights to reduce the risk of moisture.
If the rubber seal between the headlamp and the glass is cracked due to dryness, you will have to deal with moisture in your headlamp, which will cause the light pattern to clog. The reflector will rust after a while.
Check your headlights regularly for moisture and replace the seal in time, is the advice.
Mounting for vehicles up to July 1967 : the smooth side of the rubber should press against the headlight lens, the contoured side allows the rubber to rotate around the lens. Dry fit the rubber around the lens. If you use a lubricant, it is very difficult to turn the rubber around the glass.
Mounting for cars from August 1967 : The ribbed side comes against the headlight ring and do not use lubricant for mounting.
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