The reason I wondered if it was a jasper type is because I thought I saw someone post similar looking marbles but which may have had a different origin than jaspers.
I should check that out, huh?
These are some of the bottoms I have pictures of. (And the inside of one top.) The first is an illustration from the letter introducing Imperials in January 1929.
(click the links below the thumbnails for the bigger photos)
http://i119.photobuc...erialFromAd.jpg
http://i119.photobuc...o/MVC-010S1.jpg
http://i119.photobuc...ro/MVC-011S.jpg
http://i119.photobuc...ig_mc7055_b.jpg
http://i119.photobuc...orVersion-2.jpg