There is a spring and bevelled brass(?) buttons on the lower pivot;
Items 43,44 and 45
This is from the later MK11/‘S’ but i would e confident that they use the same design
Mark V Spare Parts Catalogue revised edition 1958 lists “BD.3789 Pivot, Friction, Assembly” for Front Door Ventilator on pages 67A and 67B revisions. The upper pivot assembly is listed as different part number so I take this entry to be the lower pivot assembly.
There is a shaft that fits into a brass clamp that can be tightened by a nut. To reach it you remove the wood trim but you do not have to remove the panel.