Richard Massey has sent me the following three pictures showing how the regulator engages with the glass:
The glass is clamped firmly into a metal channel piece with a rubber gasket:
The glass channel has a lower slot that the regulator rides in as it raises and lowers the glass. This lower slot has a cut-out part way along which means the regulator can be disconnected from the glass channel while the glass is still in the door. However as shown in John Maguire's picture further up it's easier to raise both regulator and glass so the top of the regulator protrudes from the top of the door, then the glass with its channel can be slid off the regulator:
Detail of one of the two bobbins at either end of the horizontal strut of the regulator, which slide in the glass channel as the regulator raises and lowers the glass. These bobbins do not rotate so lubrication is essential:
To get the regulator in the door collapse to this shape ...