There are at least 5 different ones that I know of....I have 3 of them currently to measure. There are lots of differences between them. As you found out, the spacing is different on some of them. Also, the graduations on the gauge go from 4 on the early ones, to 6 on the next series, to 9 on the later ones. The width of the gauge varied too, from 3/4" wide to 1/2"wide. The ones with the tube were meant for models with hard to see timing marks, like some trucks, vans, and industrial use. They used a special timing light that plugged into that tube.