This is Sowerby round salt pattern 1120, shown on page 41 of pattern book XI (1885), where it is indeed designated as from a registered design.
It is, as Anne says, from Sowerby RD number 294376 or 294377 (both registered on 10 September 1875 - Parcel 6)
Sowerby round salt 1123 (shown on the same page of the pattern book) bears the same registry date lozenge (so is also from RD number 294376 or 294377). There is also an oval version of this salt which is pattern number 123½.
At the moment, I can't tell you which of these Sowerby pattern numbers corresponds to a particular RD number of the pair but, if Paul S. happens by this post, he may be able to provide us with the definitive answer by reference to his collection of Sowerby design representations.