I've not seen butter in rounds, Anne, but the finial is very impractical - I can imagine it being tricky to hold it with buttery fingers, it's not easy even with clean ones, the lid tends to slide from my fingers so I'm not going to risk it!
The same finial was used on a cheese dish, a covered sugar, a honey pot, and another covered container (all on page 25), but not trinket set pieces in that catalogue.