The S3X is what this bike uses.
The two lower gears being reductions from direct drive is something shared with the old ASC hub. That was a hack of the FC (or the AF) with various pawl and ratchet sets replaced with positive drive - in fact, as the original ASC shifter is far rarer than the hub, many people just use a 4-speed shifter, missing out "2".
There was said to be a fair bit of lash between driving and braking with the ASC, and I doubt the S3X is any different. Still, if it means you can leave the rear brake off...
The Clubman looks pretty good to me, but the hub brake is pointlessly heavy and it doesn't need a derailleur eye. It's cheaper to restore the real thing, anyway:
This one is running an AM rather than an ASC, but it is a 50s clubman frame.