Green’s Dictionary of Slang

range n.

(Can./US prison) the open area outside a row of cells.

[US]A. Berkman Prison Memoirs of an Anarchist (1926) 124: I eagerly listen for the familiar swish-swash on the flagstone of the hallway: it is the old rangeman* ‘sweeping up’ (*Prisoner taking care of a range or tier of cells).
[US]Goldin et al. DAUL 174/1: Range. (Western prisons) A gallery or a tier of cells.
[Can]R. Caron Go-Boy! 146: The three of them watched me very closely, always fearful that I might do the dutch by plunging off the range.
[US]Bentley & Corbett Prison Sl. 7: Range A row of cells in a cellhouse.
[US]Other Side of the Wall: Prisoner’s Dict. July 🌐 Range: Tier, as in ‘yell down the range.’.

In phrases

flash the range (v.) (also flash the gallery)

(US prison) to scan the area outside one’s cell by using a hand mirror to catch any reflections of approaching warders etc.

[US]Maledicta V:1+2 (Summer + Winter) 267: An inmate will flash the gallery or flash the range by using a shiny object to look for the reflections on it of any guard while he is doing the illegal boiling.
[US]R. Klein Jailhouse Jargon and Street Sl. [unpub. ms.].