Whereas
5 0 refers to the police generally, Johnny 5 0 is slang for a specific cop- as in,
the 5 0 are on their way to arrest you, but Johnny 5 0 is the guy who actually puts you in those
cuffs.
I was walking down the street
minding my own business, when Johnny 5 0
popped up outta nowhere and asked me for my ID. Naturally, I got the fuck outta
dodge as fast as I could.