The bike is an MX, not a DT.
I know this because I've seen it, but just from the pic, you can see the following parts that identify it as an MX rather than a DT:
rear guard mounts.
swing arm (no holes for pillion pegs).
side cover, and mount on the frame.
tank.
lower triple clamp.
front hub & rims.
seat.
None of these are impossible to reproduce/swap, but they're all a pretty good guide that the bike is the real deal.
1973 = MX
1974 = MX-A, YZ-A, YZ-B.
1975 = MX-B, YZ-B.
I guess that we should copy the Porsche guys, and talk about the 1973 model as being the "pre-A".