To give you an idea what to look for. The original is the Smiths which you find in the second picture in the middle of this:

http://www.oldclassiccar.co.uk/car-heaters.htm

Hope this helps.

Greetings
Frank

By the way: The Barrelback is under a never ending resurrection. At the moment I'm fiddling with the electrics ;-)

1967 4/4 1500 Comp.
1935 Super Sports Barrelback