Hi Rob! I've not seen Shelley with the Copyright Design stamp before (as far as I can recall) but I'd guess that it is legit, just later than the one with the Reg Des no on it.
Designs were registered for a set period of (I think) 5 years and could be renewed up to a maximum of something like 25 years (I think from memory) so the earlier one would have the reg des number on it. The reg design might relate to the shape of the item or the method of decoration etc...
The copyright stamp protects the artwork design itself, and I am pretty sure will indicate that these were produced after the reg des protection had expired.
I'd suspect that the reg des might be for the shape of the item as much as anything as it's a less common shape and Shelley did produce various different shaped items. I have a Shelley scallop dish somewhere:
http://www.yobunny.com/pottery/index.php?topic=399.0
