If you go back to Thompson, what the book is saying is that there were two Registrations on the same date.........29780 and 29781. It has to be said, unfortunately, that Thompson's method of describing these two particular numbers could be confusing - the books says simply 'pattern and shape', and it might assumed that this was implying both these Nos. referred to the same item. There are examples of Registrations where two Nos. do in fact refer to the same object - one might well be for the shape and another for the pattern, and you do occasionally find both Nos. shown on the same piece of glass. However, that is not, apparently, the case with 29780 and 29781 - and these Nos. refer to entirely unrelated objects.
If you look at Neil's very interesting and helpful site here.......https://sites.google.com/site/molwebbhistory/Home/registered-designs/derbyshires-unregistered-pressed-glass
............... under Molineaux Webb, you'll find the explanation for each of these Registrations. No. 29781 being the pike flower trough, and your 29780 being another flower related item - although you'll be surprised at the small part your piece plays in the whole design.
You'll also see the explanation of why your item has 'hints of blue opalescence' - this is pressed glass at its best
Possibly the reason you failed to find the Reg. No. originally, is because you were looking at the earlier series of Diamond lozenge Nos. i.e. those Registrations between 1842 and 1884. Remember - if you can see a number on your glass then your item is going to be Registered after 1884 - pieces registered prior to January 1884 will show a diamond lozenge and do not show the actual No. - it needs to be decyphered.