Hey ho, 
I'm embarrassed, I broke my 3 hole flanged switching headlamp bulb when I went to inspect it.  I thought is was a twist bayonnet type and was wrong and somehow didn't twist it quite right.  Of course I can replace it, but it would be good to be able to have more options, has anyone MODIFIED this to take other types of bulbs or is there a simple way of upgrading?  (I had blown low beam but now have nothing obviously)  
I was thinking of putting a 6v LED in there as an upgraded replacement but I very much doubt there is a 3 hole flanged 6v led out there.  Wondering how you guys are doing this - silly me, now wandering in the dark
 

I feel your pain & never like changing bulbs in those. Is it just the rubber retainer or something with the pins. Would the rubber hang in there with a hose clip around it if ya totaly stuck.

I think that the bulb that holder takes is referred to as APF (American Pre Focus), so search under this if looking for a LED replacement. Note that the holes only line up in one position, even though at first glance they look equidistant. Also, you need to push and twist at the same time, with equal force on each pin/spring, otherwise two points will locate, but not the third.

That rubber fixing collar is obviously broken and the red cover is non standard - is it there to hold everything together?

Pattern bulb holders are available cheap but, in my experience, come with Honda wire colours and not the correct terminals. This is no real problem though, as you can cut the terminals off both holders and put the new rubber ring on the Yamaha bulb holder. However, you will need to fit new terminals to the Yamaha one.

As it happens, I asked the UK supplier of the pattern holders if they knew who made the rubber rings, but no luck. It seems crazy that you have to buy a new bulb holder, just because the retaining rubber ring has gone. Maybe something for Dave at KDI to look into?

US bikes had a sealed beam.  It would be nice if we had a replaceable bulb. They were not allowed here because of potential reflector corrosion plus beam focus and pattern concerns.

I strongly suggest you replace it with the correct bulb designed for that holder/lens.  The bulb filaments must be located with extreme precision in the reflector/lens or the light pattern will be unusable and difficult to see anything by and perhaps even blind oncoming drivers.  Any old bulb will not do if you want to actually be able to see anything at night.

"US bikes had a sealed beam."

When did this law come in Mark, about 1970 or so?

I've got early CT/DT and DS headlight lenses that have the replaceable APF bulb fitment.

I'd tentatively suggest that sealed beam headlights had a part number suffix of -60, and replacement bulb facility had a suffix of -61. This is certainly the case for the original DT1 headlights, but who knows with Yamaha!

My '73 H1 does not have sealed beam. U.S. model

Sealed beams were required on new autos here starting in about 1940. 

Technically, motorcycles did not fall under the same Federal Laws as automobiles and in the 1970's, 7" and 5 3/4" halogen non-sealed beams were sold here (Cibie and Hella brands were common) with "off road only" and "motorcycle use only" notations on the packaging. 

Some states adopted the "auto" laws and specified sealed beams for all vehicles including motorcycles under state laws...  I researched this years ago and want to say "early 1970's".  Pretty sure Yamaha went to sealed beams for the Enduros pretty much by 1970...  Yamaha also installed turn signals in 1971 even though the Feds didn't require them until 1973, my understanding is some states required them earlier.  I didn't look it up again, but from memory Nevada is one of the states that required turn signals in 1971.

Feds only required one mirror...  but some states like Nevada required 2 mirrors so bikes sold here came with 2.

You can install an earlier OEM non-sealed headlight in a later Yamaha, that doesn't mean it came with one.  My Suzuki and at least one of my Yamaha's have non-sealed factory headlights from earlier models...  for Yamaha at least, that option really isn't available here when you get into the monoshock era unless you use an aftermarket headlamp or source a factory Yamaha non-sealed beam from another country. 

Hey guys thanks for the info. I came to much the same conclusions as you all. I gave up with the idea of trying to mod the connector to take another bulb and have sourced a R35 15D 6v/35/35 with the apf 'weird' base (designed to encourage you to break bulbs? agh) the other idea would have been to just change the reflector or bucket to something that already has a H4 shape holder (160mm i think) . I suppose in a pinch your could put on an H4 plug and tape the bulb to the reflector.
anyway for now the American Pre Focus wins the day!

On your model you can use a very common 5 3/4" H4 lens and reflector with a 6v H4 bulb. Simply clips into the bucket rim.