Most of the pre-'69 Yens are tough in AU and higher. The '64 and '65 are a little easier and the later dates than these can be found in AU. As with most moderns Krause is worthless. Their prices don't reflect supply or demand. They try to strike a balance since the demand for moderns is so weak but then ignore the fact that buyers are highly motivated to find some of these coins. I never run into any Gems before the advent of the mint sets in 1969 and note these mint sets are extremely low mintage and command large premiums. Mint set prices in a very real way reflect the demand for the later coins that are more easily found.
I'm not sure 'key date' means as much in modern coins. Some years had mintages in the billions, others under a million. Even so, the 456,000 of 2011 is still a lot of coins.
