In English, we make a distinction between a welt pocket (which uses a flat, folded piece of fabric to finish the edge) and a piped pocket (a piece of cording is inserted into the folded piece of fabric to make a rounded edge.) Due to the difference in how the technique is viewed in each language, there are multiple translations possible either FR>EN or EN>FR. In this case, I would go with the above.

Poche passepoils ouverture 3/8
http://ballin.com/Fr/Resources/Anatomy/AnatomyPockets.html
3/8 welt pocket
http://ballin.com/En/Resources/Anatomy/AnatomyPockets.html

un passepoil = un liseret (corde, tissu, cuir) utilisé pour border et souligner une couture, une poche, un ourlet,....

I think poche passepoilée would be a welt pocket, see here [links to another site which shows a double welt pockets on the back of the trousers]
http://forum.wordreference.com/showthread.php?t=1242327

A Besom Pocket is a style of pocket. It is a "set-in" pocket (the pocket opening is cut into the garment rather than applied - "set-on" - like a patch pocket) that is finished and strengthened with "piping" or fabric "welts" along its length. It is also known in the American clothing industry as a piped, bound, welt or "Reece" pocket for an automated sewing machine that attaches the welts, pocket linings and cuts the opening in one operation. A Besom Pocket may be a plain opening or have a pocket flap, a button and buttonhole closure a tab or other functional or decorative features. It can be made of matching fabric or contrasting fabric, color or trimming and have an eccentric conture as on Cowboy Shirts or jeans.
http://en.wikipedia.org/wiki/Besom_pocket

slit pocket
Français (French)
n. - poche passepoilée
http://www.answers.com/topic/slit-pocket

Pfaff
Single welt pocket
http://www.pfaff.com/at/8137_8296.html
Poche passepoilée
http://www.pfaff.com/fr/8137_8296.html

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NITRO SNOWBOARDS | Outerwear | HOW MUCH MORE

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NITRO SNOWBOARDS. RAIDEN BINDINGS. L1 OUTERWEAR. Find a Nitro Dealer near you ... Poche pour les mains "doubles" avec rabat anti-neige. Jupe anti-neige fixe ...
www.nitrousa.com/fr-US/outerwear/how_much_more - Cached
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Forum VIP Extrême Carpe / Sweat shirt ECC !

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Vous avez été nombreux depuis un an à flasher sur les sweat utilisés lors de la ... pourriez mettre le logo de Extreme-carpe.com en bas sur la poche pour les mains. ...
forum.extreme-carpe.com/viewtopic.php?pid=27687 - Cached
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veste L1 / L1TA Let Me Go 2010

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Détails, spécifications, prix, tests et avis des internautes sur le Let ... aérations doublées en mesh, poche pour les mains zippées et détails avec logo Lita. ...
skipass.com/guide-matos/vestes/2010/l1-l1ta/let-me-go.html - Cached

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Note added at 3 days23 hrs (2009-12-04 20:58:20 GMT) Post-grading
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You're welcome, cenek, and thank you :-)

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Note added at 27 days (2009-12-28 19:30:36 GMT) Post-grading
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You're welcome, cenek, and thank you :-)