Green’s Dictionary of Slang

knocko n.

also knocker, knockman
[the knocking at one’s door or on one’s skull]

(US black) a police officer, esp. a member of the drugs squad.

[US]Goldin et al. DAUL 119/1: Knock-man. A policeman; an informer; a complaining witness in criminal proceedings.
[US]R. Price Clockers 4: The white guy [...] looked too twitchy and scared to be a knocko.
[US]Simon & Burns Corner (1998) 16: Plainclothesmen. Knockers. Six police jump out of two unmarked Chevrolets.
[US]Simon & Burns ‘The Detail’ Wire ser. 1 ep. 2 [TV script] Western [Baltimore] knockos come around here, picking shit up off the ground.