BAC wrote:
Whilst at school, we were taught that it was incorrect to put an 's' after
the 'possessive' apostrophe in the case of plural words ending in 's'. Hence
we would put, for example, the squirrels' nuts (meaning the nuts put out for
the squirrels); not the squirrels's nuts; certainly not the squirrel's nuts,
which latter would mean we were asserting the squirrel (singular) was nuts.
As an expert in these matters, can you please advise whether that usage is
still regarded as correct?

I'm French. I was told that, taking your example, 'the squirrel's nuts'
is 'the nuts OF the squirrel'. The 's' is for 'of'. Plural would be
'the squirrels's nuts'. When I see 'the squirrels' nuts' I always think
it's a mistake. I don't teach my children this way of writting. And I
hope I'm right!