Green’s Dictionary of Slang

(US Und.) San Quentin prison, California.

1935		A.J. Pollock Und. Speaks n.p.: San Q., San Quentin prison.
1956		N. Algren Walk on the Wild Side 259: There I was [...] on my way to San Q.
1963		J. Blake letter 5 May in Joint (1972) 208: He played a benefit at San Q., which gassed and chilled him at the same time.