Heres a couple of pics of the bike. The tank is an aftermarket job. But the numbers are exactly what on the Steering head and engine block.
I have had a few DT250s around the 74-76 era. However Ive never seen one with this type of head. It also came with an MX head and exhaust. Both heads are differents to the point as the MX head has the fins cut down on the opposite side of the head as the one on it.