Papajam,
I believe your question arose because of technical evolution, not regulation or marketing. It appears H4 twin-filament light bulbs were not manufactured in large quantities before 1971 (see
http://www.hellausa.com/maineng/NewLook/about/1961.html). So, there may be a transition period where early cars had H1 lights and later cars had H4 lights, possibly available as an option first before they were adopted as standard equipment.
Ruedi