French Question

I'm really glad that I walk around with so much stamp stuff in my head. Otherwise my last 16c would bounce around as I walked and I couldn't hear a thing....
But here's a thing I don't know yet. Something I see often in photos of older French de Luxe proof sheets and in the margins of some French postage stamps is a set of odd punch holes. The center hole is a flat open C form, like a drawer pull, flanked by two square cut diamonds. Here's an example from a sheet margin.

What are they called and what did they do ? My first guess would be some sort of security device. Thanks in advance for informing me.


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