Green’s Dictionary of Slang

(US Und.) a small, solitary cell; thus at the springs, confined in a strait-jacket.

1912		D. Lowrie My Life in Prison 39: In answer to my hurried inquiry about ‘the springs’ he informed me that he referred to ‘the hole’.
1924		G. Henderson Keys to Crookdom 418: Springs. At the springs – laced up in a strait-jacket.
1935		A.J. Pollock Und. Speaks n.p.: The springs, the dungeon.
1952		C. Hamilton Men of the Und. 325: Springs, A small, solitary cell.