Thats exactly right. Backspacing is the way that "mag" wheels (of which most were aluminum, not magnesium) were measured when they first came on the scene in the early 60's. Offset became the popular term in the late 70's early 80's (although it had been used for some time in Japan and Europe) due to the skyrocketing sales of Japanese cars. As the mass produced custom wheel market exploded during the 80's, the companies producing most of the wheels were from Japan or Europe, both of which use the offset formula for measuring. It's only the small handfull of American wheel manufacturers that still use the backspace method. American Racing, who's wheels are manufactured overseas, use both measurments depending upon the application of the wheel.

If you haven't figured it out already, backspacing is the measurement from the inside edge if the rim to the wheel mounting surface and is always measured in the US system of measurment. Offset is the measurement from the rims centerline to the wheel mounting surface and is always measured in millimetres.

Chris