We have confirmation from the 2022 Guidelines and Standards for Tactile Graphics!
(Though not yet published, the text of the 2022 GSTG has been approved by the BANA Board and is now being carefully prepared for general distribution.)
Regardless of the text's grade level, the numeric indicator is not used before a number that occurs below the number line. The numeric indicator is, however, used “within” a number that occurs below the number line.
So the UEB Math/Science transcription recommended by the Guidelines and Standards for Tactile Graphics for a digital time label below the number line would be c3#ae (3:15)
The numeric indicator is used within that number because of Rules of UEB 6.3.1.
Please let us know if you have any follow-up questions or concerns!