Macsub

Macsub is among other things a subtitle format.

Subtitling program for Mac?

Format

/2447
You should come to the Drama Club, too.
/2513
/2513
Yeah. The Drama Club is worried
that you haven't been coming.
/2594
/2603
I see. Sorry, I'll drop by next time.
/2675

Lines starting with a slash mark timing, seems to be frames.

A line with timing (start), one or more lines of text, line with timing (end).