now who's a clever lady - you're not a Dame for nothing Anne
Why didn't ordinary mortals like me think of the possibility of an Earthenware (CLASS IV) design.
Not being a collector of ceramics, I know little or nothing about U.K. Class IV during the period in question.
As most people know, the British Board of Trade separated entirely the various materials - glass, wood, ceramics etc. etc. into material specific groups during the period 1842 to 1884, and as can be seen from my comments above that since this Rd. No. refused to correspond to the date shown, I'd assumed therefore that probably it wasn't a U.K. Registration... shows how wrong you can be.
Since we can't see this design on the pc screen, it'll need a visit to Kew to determine whether or not this dish represents the entire item being Registered, or whether there was perhaps a lid/top, as was the case usually in the C20 for butter dishes. It might have been the case that in 1864 butter was put out onto the table fresh for each meal, and they didn't see the need for a lid.
Do people know if all earthenware Registrations - during this period - were marked with an oval as shown on this dish, instead of the lozenge (as in the case of glass - CLASS III)?
Again, sincere thanks for your sleuthing Anne - and I will comment further here after I've been to Kew and photographed the original image for this design.
P.S. meant to request please that item is now removed to British.