OK, Slithy's given us a source at least. This is what I believe the rule to cover based on what we've posted here today:

If you're writing the number as a word, the plural doesn't require an apostrophe;
According to Slithy's source the numeral will require an apostrophe (and possibly this means that 1800's might be perfectly acceptable according to
Slithy's source)
And individual letters of the alphabet should have apostrophes for plurals, again according to Slithy's source.

So p's and q's and crossing t's and dotting your i's all have apostrophes along with beats in sets of 2's and 3's...but not twos and threes and sixes...

How's that?
WW

PS: I just realized that I've repeated exactly what Slithy's already written, but I'm just throwing this all out again on page 2 so's to keep things straight in m'min'. What's the rule about the period going inside of quotation marks? That doesn't count on m'min', does it? I mean, it shouldn't be "m'min.'," should it?[/white}