Advance timing light

I found a sweet deal on a Mac advance timing light at a pawn shop. In good shape with a case, but no instruction manual. The advance dial has two sets of #s. The inner ring of numbers that form the scale around the dial are half the number of the outer ring.

OK, so why the two sets of numbers. The other advance lights I've seen have only one row. Is one for 4 cyl. and the other for 8?

Don't know, but hook it up, check your timing with the knob set to zero, then turn the knob until your timing mark shows you're firing on zero, and the knob shuld be pointing to the number your timing is set on by the timing marks on the engine.