What usage do left tortoise shell brackets (〔) play in plays? The beginning of Press-Ganging Able-Bodied Men《抓壮丁》has the following passage filled with three consecutive lines starting with left tortoise shell brackets (〔) :


  


and transcription:


  〔中秋节前一天。
  〔一个暴发户，货高利货，大斗进小斗出，盘剥起家的地主的堂屋。（如在广场演出，只要一张桌子，四根板凳或椅子就行了。）
  〔开幕时，李老拴坐在桌前抽水烟，一边唱着《江油关》。


I find it especially interesting that the brackets don't close, i.e.: you don't see the opposite facing tortoise shell bracket (〕) close off the sentences.

What role do these "〔" brackets play here?
What you are reading is a play script.

It is one of the common ways to mark non-dialogue lines in the script with "〔" or "[" ahead of the line. So in this case it's not actually functioning as half bracket, but rather just a head mark.