12.5a Numbers followed by letters.

To alert the reader that numbers have stopped and letters have begun, a letter sign is used when a letter, a letter grouping, or a hyphen and letters, immediately follows a number. Examples:

46T 78gx
46-T 78-gx

Note: An exception to this rule occurs with ordinal numbers where the number is immediately followed by letters or a part-word contraction, as in 2nd and 4th. Because context makes the meaning clear, a letter sign is not used in ordinal numbers. [VII.29]

Remember that a period terminates the effect of the number sign. Thus, when a letter immediately follows a number and a period, a letter sign is not required. Example: