3 Litre Bentley 1924
Vanden Plas Open bodied Tourer

According to Michael Richardson's book Bowler Hats and Kinky Boots. The Unofficial and Unauthorized  Guide to The Avengers, (2014) the car, which was first registered in May 1924, was exported to the United States during the 60s or early 70s.
[YT 3942][HOME][YK 6871]
Licence Plate: XR 6056

Chassis number: 544
Engine Number: 549
Coachbuilder: Park Ward
First owner: Major Yerburgh

Information from vintagebentleys.org

The car appears only twice:

the first  time in The Hour That Never Was, where Steed has an accident as he avoids a collision with a dog.
The second and last time it can be seen in The Man-Eater of Surrey Green.

Steed's Bentleys
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The Website Vintage Bentleys.org reports, that the car arrived in April 1969 from Devon in The United States. They present a photo from the unloading of the car and a further photo with the car in front of Mount Hood, Oregon from 2006.