I am looking for a way to give a traditional steel bike an adjustable stem so it fits more easily in a low bike cupboard.

I think the slotted steerer plus stem clamp arrangement seen on Twenties and other 1960s-80s folders may work, but want to check I understand how it fits together properly.

Leaving aside the nylon bushes, am I right in assuming it's basically a normal threaded steerer headset but with a larger than usual gap between the top of the head tube and the top of the steerer, so the clamp bears onto the threaded steerer above the head tube to hold the stem inside it, and also acts as a spacer to keep the headset together?

The bike I want to potentially mod is a 90s Raleigh Caprice which happens to already have a slotted steerer. It also has a quite chunky spacer but it's not as tall as the Twenty's clamp, meaning I need to find either a less tall QR clamp or a headset that frees up enough height for a Twenty clamp. Luckily it's a late enough model to be standard threads. Worst case is I buy a new 1" fork, cut the slot myself and hope the threads still work, or see if there is a way to solve this by switching to threadless fork+ahead.

Have I understood the Twenty-style headset properly? I used to have a twenty but never dissembled it, and it was a long time ago.

As the steerer is 1" I assume the clamp is also 1" and can't be swapped out with a 28.6mm seatpost clamp?

I don't remember all the details, but I recently replaced the headset on my Twenty with a 1" threadless-type headset. To clamp the stem onto the steerer, I used a shim specifically made for going from 1" to 1-1/8". (1-1/8" equals 28.6mm.)

Got Dosh ? Chris King Grip nut headsets, are made in 1.0" and 1.125"*

I got one* on my BiFri, they get the top nut made to thread down past the end of the fork steerer..

a Locking Collet of SS, holds the adjustment Solidly.