On checking back through my reference photos I find that I did have photos of a 'tricorn' dish near identical to that shown by Roy. The only reason that I hadn't posted them was that the lozenge was indistinct (apart from the fact that it appeared to indicate a registration date in either June or November 1876) - so problem now seemingly solved in that I agree with you that the 'curly' or tricorn dish is indeed from RD 301312 of 20 June 1876 - Parcel.
I attach photo of a Sowerby pattern 1165 pin dish along with its pattern book image (from page 2 of pattern book IX, 1882). There is no lozenge mark on the dish, but the centre of the dish bowl has a large peacock mark as a prominent part of the design, so the design would appear to be unregistered. It is fairly obvious, I think, that this pattern 1165 pin dish an 'uncurly' version of Roy's tricorn dish, even down to the three small feet, despite the centre pattern of the dishes differing in detail.
It is pretty evident that Sowerby used elements of this 'basketweave' design in several of their patterns (including 1159, 1175, and 1166). I suspect also that this 'basketweave' design was the basis for their unbiquitous unregistered pattern 1102 plates and dishes.
Fred.