I'd check current & voltage at the plug, check the lamp unit is held firmly so it doesn't rattle but isn't grauched in, and then assuming all is okay refit a new one.
You may also want to check the voltage regulator output in case it's going too high and popping things.

Sealed beams are like any other lightbulb, ones that fail instantly or go on forever - if a third pops quickly then something is amiss.

If both low beam headlights on my Mini Cooper stop working, I usually start by checking the fuses and relays, as these are the most common causes. Then I inspect the headlight switch and try replacing the bulbs to rule out burnt-out lights. If they still don’t work, I suspect a wiring issue or a fault in the control module, which means it’s time to take the car to a trusted garage. It’s just like playing Geometry Dash Spam Test—one small issue can throw the entire system off.

You probably won't get any answers here. Try the mk1-forum.

1963 850, renewed wiring loom when restored.

low beam failed on passenger side, high beam was still ok.  changed the whole sealed beam unit for a new one.  worked fine for three weeks - now the same side has failed  on low beam again.  coincidence?  maybe - but a bit odd.  any reason why this should happen - too much current?  how to test

many thanks in advance 

barry new zealand