The Black one is a Duncan Mosquito which shares the same body style but different internals. As far as I know theres only 1 Flying Squirrel design, but at least two Duncan Mosquito mold designs. Ironically the Duncan Mosquitos that have screw caps and can be taken apart don’t have the moldings to put the steel weights. The Mosquitos with glued on caps have the same internals as the flying squirrel on the body itself and weights can be added.
The Flying Squirrel has 3 steel balls under each cap if I recall correctly.