So I acquired this little cup, which I originally thought was a custard, but am now thinking it's a mustard missing it's lid.
It looks like pattern 1380 in the 1895'ish pattern book. Plus, it has the lozenge for 10th March 1879, which Cottle's Sowerby book lists 333170 (for that date) as a mustard with cover.
When I checked it has a lip on the inside to accommodate a lid.
Measurements-
2.5" tall
1.75" diameter at base
2.5" diameter at top
3.25" width including handle