I don't routinely have Board of Trade images for Registrations after the end of the lozenges in Jan 1884 Fred, so off the cuff can't provide a Kew picture for this item.
Cottle, Slack and Thompson all include this Rd. No. in their sequence of registrations, and although - as usual - Slack doesn't provide a description of use - Thompson says "pattern and shape of butter", whilst Cottle says simply "Butter Dish".
However, I'd suggest these descriptions from Cottle and Thompson are misleading, insofar as we now know from the factory pattern books that 77881 would appear to be a Registration for ornamentation only. This knowledge is due mostly to the Thistlewood's publication of the factory pattern books.
I've just have a look at the Thistlewood's CD Volume Three 'A CELEBRATION OF SOWERBY', and on 'Continuation pages' 106 and 107 there are a variety of images for factory pattern 1931 and 1931.1/2 - proving that these refer solely to the decoration and not the shape.
I will try and get a Kew picture of the Celery for you when next I go to TNA - from the foregoing large amounts of information I'm assuming this Rd. No. must have been allocated under CLASS 3.