Thanks for those links, RM - they'll help me avoid contradicting myself!
It does seem clear that
berk is not well-known in the US, while
berm is little-known in the UK. So I think they should remain as rare words.
Kerb is an interesting case, because of the point you mention, RM, that it is a British spelling. In the previous thread on the word, I didn't really discuss the issue, but a while later I included the word in
a group being re-classified to rare. Most of the words with variant US/UK spellings follow a pattern:
or/our,
ize/ise,
er/re, etc. So a player has a chance of inferring the existence of a word, even if they can't remember ever seeing it. This doesn't apply to
kerb, so I think I'll leave it as rare too.
The word
brae seems to me to be relatively little-used these days, outside of crossword puzzles. In the US and Australia, it seems to be mainly used in place names like Bonnie Brae, so people may be familiar with the string of letters, without knowing what it means - or whether it means anything at all. I'm not convinced I should change this one either.