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Yes, they are even listed as an FNS 40 accessory at the bottom of the FNS 40 item page. You are great to go here.

This link here takes you to the ones for the .40. https://www.budsgunshop.com/catalog/product_info.php/products_id/75180/Ameriglo+FN609+FNX+Night+Sights+FNX+40+Black

Bud's Item Number: 75180