In " A Guide To Sandwich Glass" 1st edition 1985. The authors Raymond E. Barlow and Joan E. Kaiser devote 17 pages to overshot. Two pages are a primer on overshot and the rest photo's of objects with description and comments.
According to the authors two different processes were used at the factory. The 1st process they would spread the bits of glass [ screened and sized] on the table after marvering, roll the glass on the rod in the bits and blow the article. In this process the bits would separate into "islands" and create lines of separation the likened to roadways on a map. Very much like the Tree of Life pattern in American pressed glass. Like Michaels [rocco] photo's in this thread. In fact the authors show a pitcher with exactly the same treatment as Michaels and attribute it to Sandwich. Alas, in the comments about this particular piece they also state it was reproduced in a most expert way and exhibited in the display of overshot by Czechoslovakia in the exposition "Man and His World " held in Montreal, Canada in 1975. Evidently Czech overshot manufactured and sold worldwide at that time. Sometimes marked " Czechoslovakia" but may be very difficult to see as the stamp does not 'take" very well because the nature of the glass.
In the second process they would expand the parison then roll in the bits of glass, reheat and finish the article. Thus you get a more even effect.
In the "how you tell the difference " section they state that Sandwich is lead glass and probably English overshot as well. I assume any French pieces also. By implication any Czech/Bohemian pieces are probably non lead glass.