Barbara Bain's tight belt meant the commlock stuck into her hips. In early episodes she tucked the commlock closer to the front of her stomach. When she sits, her commlock shifts sideways, almost falling out.
Starting in Ring Around The Moon, Helena's commlock hangs further out from the belt, and much lower. As these shots below shows, behind her commlock is a strap around her belt. The strap has a hook, into which Helena slots the commlock clip.
When the commlock clip is used, the top of the commlock is level with the top of the belt; normally it is higher.
She doesn't wear it all the time- when she has to remove the commlock there is no clip (as in Alpha Child, under control of Jarak, or in The Troubled Spirit when she is frightened by the ghost in Medical). When she loses her commlock in Mission of the Darians, the clip disappears too.
This is Barbara Bain's belt (more pictures). The ridge where the clip hooked on to the belt is still visible.
The problem with this hook is that when Helena walks, the commlock is much looser and can visibly wobble. This sequence in End of Eternity reveals her commlock clearly shaking loosely from her belt.
In The Infernal Machine, Helena even wears the clip with her spacesuit belt.
In The Full Circle, Koenig also wears a clip while on planet Retha.
Koenig is also wearing it with his spacesuit in War Games
Koenig and Maya both have commlock clips in The Rules Of Luton
When seated, the commlock must either be pushed to the side, as Bill Fraser does here, or a clip is used,like Maya in The A B Chrysalis.
In The Immunity Syndrome Jenny Cresswell has the belt clip.
Copyright Martin Willey, thanks to Simon Rhodes