Do you read Japanese? If so general advice would be to spend some time in the "leaflets" section on Yahoo Japan Auctions. I've found that with posters, sometimes the original Eirin mark was re-used on re-release posters. However, the leaflets/chirashi and also the pamphlets sometimes have other info printed on them that can offer clues. Sometimes other films are listed, or dates, or advertisements, company logos, etc...
Regarding this particular poster keep this in mind:
Japanese B0 = orange (matte paper)
Japanese STB = orange (matte paper)
Japanese B2 = orange (matte paper)
Japanese B2 = blue (glossy paper)
Now, to me this means the orange was DEFINITELY the first printed. That pretty much leaves two possibilities -- 1) maybe interest/word of mouth extended the distribution and they printed a second run on blue glossy, or 2) there was a second release and they used same plates/Eirin, but printed on glossy stock with blue ink.
Either seems plausible. In the world of Easy Rider leaflets -- there are three variants:
1. Orange art
2. Orange and Blue art & double billed with 'Cactus Flower' -- definitely from 1969
3. Blue art
The leaflets were usually stamped with local/showtime info. Here are three variants of the orange one. I can't read Japanese but maybe these might help?
For some reason all the orange copies I've seen were stamped on the front on the top of the art, even though there is a space for it on the back!
The blue fliers all seem to be stamped on the back.
For completeness sake, here is the double bill flier with Cactus Flower that uses the orange AND blue art:
And by the way, both the B3 press sheet and the brochure use the combined orange AND blue art.
What does it all mean? You've got me. There definitely aren't a lot of glossy stock posters at that time period, but there are a few big titles for sure (Bullitt, Le Mans, Godfather, Enter the Dragon, etc...). If anyone can solve the mystery I would appreciate knowing the answer too!