What's the answer?

[Diablo007]

I may be way off base...  I seem to recall certain marques making engines that would allow you to switch the shift mechanism from the left to the right, or vice-versa?  this was when standardization of controls was on the horizon.  Might this be the reason for the differences?  I think the YR1 was one of these and we have one at the shop.  The next time I'm there I'll see if I can get to it (it's pretty deep in with a bunch of other motorcycles in front of it) and take pictures of the cases to use for comparison.

On a similar (control differences) note, we have a '40's Indian 4 that will be going up for sale.  Anyone interested?  Black with tan seat.

[DesmoDrew]

   The Left or Right shifting was an  option for a couple of reasons and mostly available on 'race' inspired models.  All early Yamaha TZs came with a double ended shift shaft installed (a nifty rubber boot covered the right side splined end).  This was so European racers could shift on the right, since early British and European bikes had Right side shift, and that is what they were used to.  Sheene and Agostini (and many others) used Right side shift in their GP careers.
   Yamaha DT (and other off road singles) engines also have the option of Right side shift.  This is because so many were used for flat track racing, and those bikes all shift on the Right side, for obvious reasons. Any why some factory heads have Two 'spark plug' holes.  The extra hole was never meant to carry a spare plug, as many think.  It is to install a compression release for flat track racing.  It was unique sound to hear a bunch of two-stroke singles heading into the first turn of a flat track race and everyone hitting the compression release...

[1976RD400C]

One of my favorite old pics I took as a young teen. This was an AMA sanctioned flat track race in 1970. The guy in the middle has a compression release on his Hodaka 100. Check out the riding attire, gloves?, boots, sweatshirts. Check out the spectators getting their ears blown out by the open chamber bikes getting ready to rip. 

[m in sc]

lots of bikes from the era had that 'option', or capability. my t500 has the shaft both ways, and the 74 morini 3 1/2 i had restored had a us market only factory series of linkage to move it from the rh to the left for us market only. rare bird that was.  as a side note, both these bikes have cable rear brakes and both had the ability to swap the sides of the brake as well. 

[Striker1423]

A buddy of mine had a friend that passed. His friends wife sold off his bikes. One was an AMF Harley Sportster in immaculate stock condition. I wanted it bad, as she was selling it for $1500 bucks, but it had right side shift and I knew between the two bikes at some point I'd get confused. 

Apparently those AMF's are hard to come by in complete stock condition like that. Whoops.