a6.1. Names ending in -ch, -es, -s, -x and -z take es as plural (the Jameses, the Asterixes).
a6.2. Single letters require an apostrophe to form their plurals (Annie got three A's on her report card).
a6.3. Multiple letters add only an -s to form plurals (Annie got three INCs on her report card. The RAs are planning six programs this year.)
a6.4. All numerals add only an -s to form plurals (the 1980s, Boeing 757s, temperatures in the low 40s).