The Universal rule (i.e., American English and British English) is: don't add -s when the word ends in an -s, like this,
Cyrus' car :D
Spectres' existence :D
Vessels' peace :D
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I notice there are differences between US English and UK English,...
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Yes. It's called
the greengrocers' apostrophe. Click
here.
All the best, :D