As MPWR has stated, Datsun was the original name that the cars were marketed under changing to Nissan I believe in the latter half of the '70s.
But, when you say 240, do you mean 240Z or 240K as they are different bodies. The 240Z has the pointy nose whereas the 240k came as both a 2 door and 4 door car with a flat nose. The rear is also different however the easy way is the nose.
240Z
http://en.wikipedia.org/wiki/Datsun_240Z
240K sedan
http://www.baronmoss.demon.co.uk/datsun_240K_pics.html
240K 2 door
http://www.motorbase.com/vehicle/by-.../gallery.ehtml