In the world of arms and armour there are specialised words for different types of head-gear

Cap, Casque, Basinet, Helmet, Great Helm form a progression in coverage from a simple skullcap to a full immobile enclosure. There are a bunch of other spealised terms, too, but these are enough for now!

The Russian designers (or their translators) appear to use the word "casque" to mean a rigid cap-like part of the whole helmet. In this context, I would probably use the word "shell".

There is an outer casque - the shell we see with the Label CCCP. There is an inner pressure layer with a visor and a flexible membrane at the back, and then there is an inner casque or shell to stop the flexible membrane rucking-up and poking the cosmonaut in the back of the head.

All quite explanatory, really.