Interesting discussion in both these threads.... and raises the question as to what exactly the terminology is, and what role it serves.
Here we have an item, which does not appear to have a proper term, a No.1 (Davidson) or a part number 1634/I (Libochovice) or whatever. We choose to define this item, using terms by which we may recognise what is meant. I agree with Bernard, that to talk of a pelican frog is ridiculous. But having looked at several centrepieces, both two-piece and one-piece, maybe, just maybe, "frog", is a corruption of frock. I ask, as it might be more appropriate, to talk of centrepiece and seperate frock, as arguably, that is what some appear to resemble, or centrepiece with integral frock. However, the OED, does give to the word frog, the definition of a support....for bayonets and swords, and that may be also why these items are so described.
Sadly, too little attention is paid these days, to recording both the coining of terminology, and its subsequent use, development and corruption, which, were it considered more carefully, would allow us to understand where, and how, certain uses of language derived, and its proper meaning.