as Iain says its a MX250, later serial number would be A model yellow, early numbers are silver tank.
when he say all the frames are the same, he means they look the same but have differant head stem numbers. the engines however are prity much all differant inside as are the barrels etc. Body work on the 250/360 is much the same the 500 has differances like exht and side cover etc.
Still 364 make great all round pre 75 bikes, and heaps out there now beign raced. There kinda bullet proof and parts are plentiful and when they run out you can just use the DT250 stuff