Krupke is addressed as both Officer and Sergeant. But those are two distinct ranks, unless the NYPD does things differently. He can't be both; he must be a sergeant, as I noticed the chevrons on his sleeve. But what's the explanation for being called Officer? Was he just recently promoted to Sergeant and the Jets haven't got used to that yet? Or do they call him Officer to be obnoxious?
... Or was there just a mistake in the writing?