You're quite right, Pat.
Pone is not common at all. It seems that even in its home country, it has a regional character. Here is a description of a breakfast served up in Georgia from the O. Henry story "Hostages to Momus" (courtesy of Project Gutenberg):
That breakfast turned out to be composed of fried bacon and a yellowish edifice that proved up something between pound cake and flexible sandstone. The landlord calls it corn pone...
I became aware of the word as a youngster because of a song that was often played on the radio at one time about a man named Jubilation T. Cornpone. But I never knew what corn pone actually was - I assumed, like Toni, that it meant the same as corn cob.
Just to add to the linguistic confusion, in England the word
corn is used to refer to any grain, while in America it refers to the product of a specific plant, known to the English as
maize. (Because Australia follows the US usage in this matter, I was quite confused for a time when reading about the British Corn Laws - why had people in England been so agitated about the price of corn on the cob, I wondered.)
So I will drop
pone as a common word, although I imagine a lot of regular players will have got used to it (without necessarily knowing what it means) because it has appeared in 48 daily puzzles so far.
Regarding your query about
roster, Pat, it is classed as common. I don't see it as a specifically US word. It has a wider meaning than
rota. For one thing, it can be used as a verb, which
rota cannot. The Shorter Oxford gives the usage example:
Two inexperienced co-pilots had been rostered to fly together.
(The quote is from English writer Norman Dixon. The dictionary also gives two quotes using
roster as a noun - one from an English writer and the other from an American.)
Rota (from the Latin for
wheel) is used for a list of people or duties on a recurring schedule.
Roster (from a Dutch word for a gridiron - related to
roast) can be used for the same meaning, but has also been extended to any list of people. Searching on the websites of British newspapers yields hundreds of uses of the word
roster. The most recent examples from
The Times, both from last Friday, use the word for a recording company's list of artists:
Listen to the tracks on her recent CD, Between Me and the Wardrobe – which eventually found its way on to the Blue Note roster – and you find yourself thinking of Joni Mitchell, Janis Ian and even the Kinks at times.
Gratuitous plugs for Sony Ericsson phones and strategically-placed posters advertising bands from the Universal roster...
I don't think
rota could be used in either of these cases.
So I don't think
roster is unknown in Britain, and I'm not persuaded it should be classed as rare.