12.5c Letters followed by numbers.

When a single letter is followed immediately by a number, a letter sign is not required. However, a single letter followed by a hyphen and a number needs a letter sign so that the letter will not be misread as a whole-word contraction. Examples:

X19 X-19

When a letter grouping is followed by a number, or a hyphen and a number, a letter sign is used only if such letters could be mistaken for a short-form word. [II.12.a(4)] Examples:

RCV90 lr-14
TX48 gyv-72
ES-18 W-2GS;
17C4-6 C22A

[Reference citations, such as II.12.a(4), that include numbers and letters separated by periods and decimal points will be studied in Lesson 17.]