With marbles this old the pontil detail can get lost with surface wear, but basically what you're looking for is evidence of faceting in the circled area.
The faceting can sometimes be so well done you need to look for reflections in bright light (similar to vintage Agates)
Based on the pattern, color combo and everything else, im confident in my original comment this is a German Greiner, and not a later American melted pontil Leighton