Steed's Cars - Alvis Speed
Alvis Speed 20 or 25 Open Tourer /Alvis Firefly?
as seen in Traitor in Zebra.

It is Steed's only vintage car in series 2 of The Avengers, he drives the car only in one episode.

Licence Plate is CPT 75

CP-series would be a Halifax issued number used between Dec 1903 - May 1932

This car gives me a headache.
The licence plate - if it belongs to the car- would be that one of a car before 1933. The Alvis Speed 25 however was a British touring car made between 1936 and 1940 by Alvis Ltd in Coventry; so could Steed Alvis be a Speed 20?
Alvis speed 25 Tourer; more photos on the site of Powerful cars
photo above by: www.history-of-cars.com
Unfortunately the colour of Steed's car is unknown, but there were several different tones of the Alvis.

- British Racing Green combined with beige leather seats
- British Racing Green combined with trim colour red
- Dark Blue body colour with grey trim colour

There also existed two tone cars:

Two tone red over maroon (trim colour?)
Two tone cream over maroon (trim colour?)

I would exclude the version British Racing Green combined with trim colour red for Steed's car in Traitor in Zebra, because his car's leather seats look light (in black and white) compared to red seats on photos changed to black and white.