Green’s Dictionary of Slang

odd n.

[ety. unknown]

1. a police officer.

[UK]F. Norman Bang To Rights 150: He was driving along the Old Kent Road when [...] the odd gongs him down.
[UK]J. Morton Lowspeak 107: Odd – the police, particularly detectives.

2. a police car.

[UK]F. Norman Bang To Rights 151: The boggie led him by the arm to the odd.

In compounds

odd lot (n.)

1. a police car.

[UK]F. Norman Bang To Rights 148: He was driveing along [...] and all-of-a-sudden a couple of boggies in an odd lot have fancied him.
[UK]J. Morton Lowspeak 107: Oddlot – a police car. In fashion at the height of the teddyboy era.

2. by. ext. of sense 1, undercover policemen.

[UK]J.J. Connolly Viva La Madness 56: He has to pick his moment, just in case hes being spied on by the odd lot.