Good Evening / Morning Ladies and Gentlemen

A question:

I have come across a gentleman who has said I can have first pick of several Western Electric No 1-A Flying Helmets - actually he has 10.....

The primary question is: each has a hole through the top cinch strap. I've seen this before, but have no idea what the significance is - if any - I have not been able to find any information on this interweb thingy, nor in books.

Only two have labels and they state: Western Electric No 1-A and the size, as opposed to the previous and Patent Pending - so I assume they are slightly later models - again no luck with the research.

Has anyone got any idea about: 1. dating them; 2. the significance of the hole?

Many thanks

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...well you get the picture.