Add the opinion letter's date

Warning²Federal courts, Illinois courts21st and 22nd editions

An agency opinion letter is cited as its agency's work, with the date it was published: an agency issues many letters, and nothing else in the citation tells them apart.¹

1. 1R. 14.2(d) Other publications. Tables: R. 15.1(c) Institutional authors.
2. 2A warning: the form is usually wrong under the rule in note 1, and context can justify it.