"USA law at the time stated that front turn signals needed to be mounted as wide as possible/practical on a motorcycle.  That's why the US Enduro models had the signals on the handlebars.  

Street bikes often had narrower handlebars so stalks on the headlight placed the signals just as wide and/or there wasn't enough room on the bars."

Again, just another observation here, and that is the non handlebar mounted front indicators were often mounted on "extension" brackets that sat somewhere around the top yoke (triple tree). This didn't apply to UK market bikes, although they can be retro fitted.

Also, Yamaha did at least three different lengths of stalk on those earlier style indicators!