They are solid ivory and known as "Burt Double Stripe"

I found a a box that was one shy of a complete set and offered them on eBay, at F.U.N. and via contacting pro pool players. Though none of the balls were plugged, some were worn to the point where the color was rubbed away. They sold for $300, $500, $700 and $1,500 for two of the best condition pieces. Note the numbers being hand engraved like on MOP chips.