I believe this headlight topic has been bandied about in the past, I would like to get some input from other folks who have restored a RT3 and especially the headlight.
What Im finding as I put my headlight back together  is that if you do not have all the oem parts from the day they were built, the light does not work as intended with a aftermarket sealed beam and or associated parts.
The oem sealed beam allowed you to use tension springs to hold light in and thereby allowing you to use adjusting screw to tilt light etc.
Also it seems the Koito outer chrome lens ring had the proper attachments to mate with the oem haeadlight.which was also Koito, hard to find parts.....
So If I use an aftermarket headlight I will have to use W clips to hold light on ring, at which point i do not think the adjuster screw functions....
One other option I have been looking at is using a semi sealed light that will take a 6 volt H4 bulb with lower wattage of 35W so system can handle. Has anyone out here ran across the same situation.
I appreciate any and all help. Maybe this is a simple fix and Im just missing a step or two.
thanks
 

Check out with the seller the dimensions of this lens - it might be a straight fit (I'm pretty sure the 12v 35/35w is for the bike it was originally fitted to and so can be ignored in this case).

It takes an APF bulb and you'll also need a bulb holder for that.

The early Enduros (and likely all models) had this style of lens, before switching to sealed beam.

1969 - 1973 Yamaha SL338 SS338 Headlight Lens 110-31905 806-84320-00 | eBay

thank you will look into