try southern pride models

http://www.footplate.co.uk/index.php?main_page=index&manufacturers_id=9&sort=20a&page=2&zenid=b5ac7ca31e255a52537cf1e7931ec6c1

Do you mean the ones converted in the late 70s or their predecessors which would have been in use during the 70s?

The ones Roy H linked to are the former, pictures are on Paul Bartlett's site and departmentals.com, the latter would have been mostly former LNER and LMS coaches, pictures do exist on the afore mentioned sites but are rarer. Here's a youtube vid with grainy pics from 15 or so years earlier