Green’s Dictionary of Slang

(US Und.) a police patrol wagon.

1914		Jackson & Hellyer Vocab. Criminal Sl.
1936		Mencken Amer. Lang. (4th edn) 577: Making an attribute do duty for the whole produces broad for woman, clatter for patrol-wagon, apple-knocker for farmer [...] and big-house for prison.
1950		Goldin et al. DAUL 44/2: Clatter. A police patrol wagon.