:)
It's a type of clip or fastener

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Note added at 22 mins (2024-03-13 14:05:23 GMT)
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Sugiro deixar no original

"presilha de canotão ou de galão dourado" would be either "bullion clasp" or "braid clasp"

I asked ChatGPT4, what canotão meant in the sentence, it returned:
The word "canotão" in Portuguese refers to a type of thick, cylindrical, often metallic decorative element used in military uniforms, especially on hats or as part of the fastening mechanisms for decorations or insignias. It can be made of metal, such as gold or silver, and is characterized by its robust appearance. This decorative element is commonly used to add a sense of distinction and elegance to military attire, making it stand out in formal or ceremonial contexts.

In English, a close translation could be "bullion" or "bullion cord," especially when referring to decorative elements used in military or ceremonial uniforms. Bullion in this context refers to gold or silver in thread form, used in embroidery or for making cords and tassels that add a decorative and often luxurious touch to uniforms, hats, and other items of clothing. The term captures the essence of the "canotão" as a decorative and prestigious element in military uniforms.

And looking at the link to military wear below, it looks like a good match as they sell a lot of bullion braid material.