As Neil will know in vastly more detail than me, Molineaux Webb Registered many pressed items in the 1860s - 1870s showing Greek Key and frosted open spaces and I can't imagine the piece shown here by Neil is anything other than a custard with handle - jellies tend to lack the handle. Despite the prodigious quantities of such utility items having been around both before and during the pressed era, I was staggered to draw a blank when looking through the Kew archives spanning the period 1840 to late 1870s - people who collect such pieces know how commonly they were produced during C19 and on into the early C20. Quite what the overriding reason was for the lack of Registered custards I've no idea.
M. & W. Registered a variety of specific shapes and designs, many showing the G/Key, frosting and cog wheel rims, the given Registrations all appearing to protect both the shape and surface design. Is it possible, as occurred similarly with some other manufacturers, that M. & W. simply took one of their already used G/Key designs with frosting, and applied it to a custard. As Neil says, in the absence of a diamond or No. then doubtless we shall never know the answer.
Mostly unrelated, but of interest re the C19 use of the G/Key on a Registered item - the attached picture, which we've seen here before, is possibly the earliest use of the Key pattern during the lozenge/Registered period. The item is blown I'm sure, and never pressed, but time wise fits with the then current passion for interest in Classical art etc.