Your creamer is Sowerby pattern number 1449, shown on page 50 of their pattern book XI (1885), with a diamond symbol next to it confirming that it is, indeed, a registered design.
I attach a photo of the 1449 creamer in white vitro-porcelain for comparison.
There is a matching sugar bowl (but it has little ball feet rather than a single foot like the creamer) though currently I don’t have a reference photo for it. .
As you say, Sowerby registered three designs on 9 January 1880: (
345042 (described by Jenny Thompson as a vase)
345043 (described by Jenny Thompson as a square trinket)
345044 (described by Jenny Thompson as a square sugar basin).
RD 345044 is actually Sowerby pattern number 1464 (also shown on page 50 of the Sowerby pattern book XI, 1885), so the pattern number 1449 creamer is presumably from RD 345042 or RD 354043. Unfortunately, I don’t know exactly which is the correct RD number for it (but perhaps Paul S. has it in his design representation reference photos).
Fred.