Chrome pancake air filter for 1.25" SU. 5.5" diameter, offset mounting, 1" (+0.5" nuts) thick. Washable, reusable filter element. Many SU carb users (stock on all Spridgets) found their car equipped with pancake air cleaners. These have been popular for many, many years.
The latest version of these now have highly polished chrome tops and bottoms. Very simple to install, they just bolt to the face of the carb. No clearance problems, no questions, just simple and easy.