CB750K built between 1969 and 1972
K what? I think from the colour this is a 1972 K2.
1972 Yamaha DS7
Yamaha DX250 (Called in Japan). It made in 1970-1972. http://bikes.seesaa.net/article/20074454.html http://blogs.yahoo.co.jp/kazu31f128564/55690465.html
Kawasaki TR. 250?, 350?... The engine displacement is not understood from this photograph. http://www002.upp.so-net.ne.jp/Topcon/Kawasakihistory/kawasakihistory.htm http://www.bbb-bike.com/history/data120_2.html
I think, it's Yamaha YDS-3, too. Manufacturing was begun as for this in 1967. http://www.yamaha-motor.co.jp/profile/cp/history/motorcycle1960/index.html