Ribble Chrono TT aluminum frame is 52cm square.
Columbus Tusk aero carbon fork (steerer is approx 185mm)
The frame has seen better days. It originally had an integrated seat post clamp which developed a hairline crack due to over tightening. As a solution I cut the integrated clamp off and filed the tubing so that you can fit a separate seat post collar. It's not a neat solution, but it worked OK. You can see this in the photos linked below.
More photos: https://photos.app.goo.gl/RMCtT9PjCZxLgomBA
I'd prefer not to post, collection from NW1 or meet up.
Dibs and PM if interested.