A manual for writers of research papers, theses, and dissertations, 7th edition - Kate L. Turabian 2007

Plurals and punctuation - Plurals
Numbers
Part III. Style 20 spelling

Form the plurals of spelled-out numbers like the plurals of other nouns (see 20.1).

Half the men surveyed were in their thirties or forties.

Form the plurals of numbers expressed in numerals by adding s alone (not 's).

The pattern changed in the late 1990s as more taxpayers submitted 1040s online.

To fly 767s, the pilots required special training.