I use a US IDP (Geneva 1949) with no problem. IDP conforming to Vienna 1968 treaties are not valid in Japan
I've had to show my IDP numerous times, IME the police do not check carefully. I would be surprised if they know the difference between the 1949 version and the 1968 version.
That said, driving without a valid license is quite a serious offense in Japan so I would not take a chance. Also, there is a real risk you won't be able to rent a car without a 1949 permit.
I believe most states that are party to the 1968 convention will also issue 1949 form IDPs as well.