This is one of Barbara Bain's year 1 belts (thanks to Lionel). It is 85 cm (33 inches) long. Barbara's name is on a strip of fabric inside the belt, above the Rudi Gernreich stamp. Below it is the faint name "Eileen Harvey", who was Barbara's double in some episodes (Last Sunset call sheet).
The belt has creases near the end which enable us to identify it in episodes. The crease is visible when it reflects lights; without lighting, it is not very visible even now.
The crease seen in this publicity shot from War Games.
From The Last Enemy onwards it can be seen until the end of the series.
The curved end of Helena's belt is slightly irregular. The top is a bigger, more regular curve, while the bottom of the curve has some flattened edges and points, where the vinyl was not cut and folded well. This is subtle, but just visible in some shots, identifying the belt in earlier episodes before it acquired the crease. We can see it in these shots from Missing Link and Alpha Child. It seems to have been used from Matter of Life and Death.
Helena watches Koenig in Guardian Of Piri. She is not wearing her belt.
Contents copyright Martin Willey. Thanks to Lionel and Simon.