A good prior question is: how do we know they are CH and SO'D, respectively? Well, the first one is signed and stickered in the regular fashion (photo C), so that is obvious. The second one has no signature and no other obvious maker’s mark (photo D). However, I know who made it because I bought it years ago from Tina Cooper's shop when it was at the top end of town in Eumundi, along with a couple of other SO'D pieces.
Some people claim to detect differences in the iridized finishes, but I find there is too much variability within makers and too much overlap between them for that to be a reliable indicator.
The very fact that Exhibit B is unsigned is itself an indicator. Very little of Colin Heaney's output is unsigned, except for the jewellery range (goddesses and pendants, etc.) and items intended to be components (architectural pieces, flowers to go in a bunch, etc.). Even lowly paperweights if intended for separate sale are fastidiously signed and often stickered as well. By contrast, almost none of the smaller Sean O'Donoghue pieces are marked at all (along with quite a number of his larger pieces). So even allowing for the significant difference in their output levels of such things, an unsigned iridized seashell is much more likely to be SO'D than CH.
These shell paperweights are made by pushing a knob of softened glass into an open mould. They are then taken out of the mould on a pontil rod and sprayed with heavy metal salts and heated in the absence of oxygen until metal precipitates on the surface. Thus the shape is characteristic of the mould and replicated almost exactly from one specimen to the next.
Perhaps the most easily recognisable difference in the shapes is found in the hinge region of the clam shell. The CH shape is almost a straight line there (photo E) while the SO'D one forms a triangle (photo F).
There are other CH moulds which produce shells of smaller diameter but greater thickness, where the hinge appears somewhat curved, particularly if the view is not taken square on. But even they are clearly distinguishable from the feature seen in photo F.
Another feature that distinguishes most if not all of the CH shells of this kind is the use of a pusher to force the soft glass against the mould. In both cases a paddle of some kind would be used to smooth off the surface that forms the back of the piece. But the CH ones also come away with a large circular indentation that occupies over one third of the area of the back, as seen in photo C. There is also a smaller pontil mark in the middle, which can be seen on the SO'D specimen too.
So there is little doubt who made the seashells by the seashore, whether in Noosa or Byron Bay.