I have seen tiny tubes like your B7, but have not used them. I have used glass filters, of many descriptions in various labs although they often came in big ceramic funnels.
I am mystified by your riddle after all!
I can't quite work out the shape - on one pic it looks like a flattened bubble, in the other, it looks as if there is a rim around the outside?
I am imagining it could be used as a flexible stopper for something that produces a gas, slowly?
The weight of the metal would hold it down, but enough pressure would release some of it, from under the weight of the metal, while preserving the "head-space" of the gas released above the liquid?
Or, it could be an early sort of flexible plug for a sink!

(Our German '70s not best, but not dreadful glass, will remain a mystery for now.

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