Hi dpendzic,
the thumb is a diagramatic way of depicting a single and double flange roller for position on the track frame.
Double flange rollers have 4 flanges--2 outside and 2 inside.
Single flange rollers have 2 flanges--2 outside only.
See scan below--double flange to the left, and single flange to the right.
Cheers,
Eddie B.

OM, I never saw a sheet with the roller procedure like that but at the Cat dealer I worked for we were always taught to install a double at the front and a single at the rear because of interference with the sprocket teeth of all roller frames regardless of what tractor model it was on. I think the reason for the double in front was to give more support to the track so on turns that the track wouldn,t jump off esp. when the track got a little loose.

Thanks guys--once i got back to my shop and looked at the rollers i realized how dumb a question on the single flange rollers was!