the 99 short rifles are probably the most commen rifles found. Maybe 6.5 m/m long rifles would be next. There is also a 99 long rifle, while not "rare",
in short supply. It is impossible to date a jap rifle exactly. You would think the series number would be in sequence but they are not and they were assigned to different makers. I have seen people make estimates of year only.
Go to ParallaxBill,s site, jap forum, for more detailed info.