Thank you for looking at the Sowerby design representations, Paul.
Definitely Sowerby pattern 1663, shown on page 52 of Pattern Book XI, 1885, confirmed as Sowerby by the peacock head trademark, of course. A matching pattern 1663 stemmed sugar basin is shown adjacent to it in the pattern book.
This is the first example of a pattern 1663 creamer that I have seen (and I have never seen an example of the matching sugar basin), so unfortunately I do not have any reference photos of the appropriate lozenge to hand for comparison.
The only two currently correlated Sowerby patterns in the #1600s from registered designs are #1672 toastrack (from RD 374774 of 18 December 1881) and #1639 suite of tableware(from RD 380133 of 28 April 1882), so it would seem not unlikely likely that the lozenge dates from a similar period.
I too have tried the reversal of the images (and making greyscale, altering the contrast and brightness, and sharpening the image), and my interpretation seems to be 21-(probably E)-I-16, giving a date of 21 July 1881-Parcel 16 (which is, again, an ‘invalid’ Sowerby registration date).
Now, the ONLY Sowerby registration from that 1881-1882 period with a date letter ‘21’ is from 21 September 1881 – Parcel 16 (a bundle covering RDs 370370 to 370379). A lozenge for that RD bundle should read 21-E-D-16, compared to the apparent Blu-Tack impression reading of 21-E-I-16 – a 75% match for the 21 September 1881 lozenge alphanumeric signifiers, and a nearish-match for the known Sowerby registration bundle to boot.
That 21 September 1881- Parcel 16 Sowerby registration bundle contains 4 sugar basins (2 on stem, 2 not on stem), 3 butter dishes, a celery, and a sweetmeat. So at least there is a possibility the 1663 sugar basin design in that bundle (and, therefore, the matching 1663 creamer).
It may possibly be that the mould-maker or lozenge-embellisher has mis-cut one of the registry lozenge signifiers (an ‘I’ instead of a ‘D’, perhaps? – or merely wishful thinking on my part?).
Ultimately, though, the most likely definitive answer is either a Sowerby pattern 1663 creamer or sugar basin with a clearer lozenge, or from another? perusal of the Sowerby design representations for the relevant period.
Fred.