I don’t know how to copy the image, so please make a search for
"fly extenstion" tirapancia
and have a look to the images

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Note added at 39 mins (2021-01-06 19:29:24 GMT)
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https://threadtheory.ca/blogs/sew-alongs/jeans-sew-along-the...

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Note added at 41 mins (2021-01-06 19:30:52 GMT)
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https://www.lanieri.com/blog/it/guida-ai-pantaloni-su-misura...

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Note added at 47 mins (2021-01-06 19:37:02 GMT)
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Imagedocplayer.it › 15146009-Visual...
Risultati web
VISUAL GLOSSARY Italian & English - PDF Download ...
... Turn-up Risvolto Hem Orlo Waistband extension Prolungamento fly extension tirapancia side pocket tasche

Tirapancia / Pancierina
I due dettagli hanno la stessa funzione, mantenere la vestibilità inalterata della parte davanti del pantalone. Nel caso di chiusura della patta con zip applicheremo il Tirapancia. Nel caso di chiusura della patta con bottoniera applicheremo la Pancierina.

https://www.lanieri.com/blog/it/guida-ai-pantaloni-su-misura...
"Linea sempre ben definita? Ci pensa il tirapancia. Il sistema bottone + gancio in tessuto cucito internamente nella parte frontale dei pantaloni, proprio sopra la patta. Permette di tenere sempre il tessuto ben teso per una struttura impeccabile all day long."
https://www.gentlemansgazette.com/features-of-a-pair-of-pant...
"The front zipper is usually a standard and therefore very forgettable detail on a pair of pants. However, more fancy designs can feature a button fly as well as an elaborate system of a flap (sometimes called a French bearer) and inner buttons to fasten up the pants"

https://www.suitsmen.co.uk/glossary.php
"French Bearer
The french bearer is a special button behind the fly of plain fronted trousers. It helps hold the front of the trouser flat, so making them look smarter. Click here for the trousers we sell that have a french bearer."

in tailoring, what is a French Bearer ? | Yahoo Answersanswers.yahoo.com › ... › Fashion & Accessories
'A French bearer' is that trouser waistband 3" extension, just above the fly, that normally clips, but can button, into position to secure a neat and tidy fit. Source(s): ...
1 answer