543 & 700
Finally had a moment to talk with Stan Kann. He did verify what Erick and others said, that the 543 and 700 both came out at the same time and the 543 was a low-tier model.
He did affirm what I thought, but was not sure, about the three 543 models:
1 - straight handle, switch on motor
2 - curved handle, switch on motor, cord runs up exterior of handle
3 - curved handle, switch on handle, cord rus up inside the handle