The frame number format varies, depending on which factory they had it built in.
I think the RMB frames were built in their own factory in China, which was established at around that time.
I think 03 means it was built in 2003 - i.e., for the 2004 model year.
I thought 07 was the code for the model, but I've also seen Vertex 70 frames with 0306 numbers, so that's a puzzle.
1250 is the serial number, but I don't know whether it means it was the 1,250th Vertex or the 1,250th frame of any model.