Douglas Harper's Etymology Dictionary
1960, in computing, agent noun from delimit.
n. That which delimits, that separates.
A delimiter is a sequence of one or more characters used to specify the boundary between separate, independent regions in plain text or other data streams. An example of a delimiter is the comma character, which acts as a field delimiter in a sequence of comma-separated values.
Delimiters represent one of various means to specify boundaries in a data stream. Declarative notation, for example, is an alternate method that uses a length field at the start of a data stream to specify the number of characters that the data stream contains.
Usage examples of "delimiter".
Text delimiters are characters that do not ordinarily appear within the text of a title, such as slash ( / ), double quote ("), and tilde ( ~ ).
Smith, when faced with this problem, decided that the easiest way to resolve this was to use a different dialogue delimiter so that the reader would instantly know which communications were verbal and which were mind-to-mind.
I think we're seeing delimiters around these two strings of twelve pulses.