Hi all, I've posted this to CW support, but as it's a weekend I thought I'd ask here too: Anyone come across this?
In case anyone's curious, the answer that came back was that you don't use an apostrophe, unless not using one would mean it could be confused with something else. In my example above it would be "M's and Ns." An apostrophe with the "M" because without it, it looks like the title "Ms."
I'm surprised! That seems oddly quirky. I'd think if they might sometimes want an apostrophe, one should always be used.