Sowerby RD 84218 was registered on 18 October 1887.
The corresponding Sowerby pattern number is 1947, and it appears both as a bowl and a 2-part comportier on continuation page 106 of the Sowerby pattern book XI (1885), so the design was registered after the pattern book was produced.
I attach some photos of a 12 inch round dish and a 12 inch 2-part comportier in the pattern, completely in accord with the pattern book. The bowl is marked, with peacock and Rd number, and the comportier unmarked.
The pattern book gives the 1947 comportier in 2 sizes – 10¾ inch and 12 inch; the round bowl in 5 sizes – 5, 6, 8¼, 10¾ and 12 inch; and an oval dish in the same pattern in 4 sizes- 6, 8½, 10 and 12 inch.
The sides of the bowl obviously differ from Bernard’s, so it may well be that the registration was for some other aspect of the design. I notice that Thompson describes the registration as "pattern of dish (imit.cut)".
I have searched the extant pattern books for a 2-part comportier with cut sides similar to that 11½ inch two-part comportier launched PG 1 Oct 1887 (Cottle p20), but to no avail.