Replacement blade retraction button for a utility knife.  Fits this Home Planet knife: https://www.amazon.com/gp/product/B09DDLCT22/ref=ppx_yo_dt_b_asin_titl

May fit other knives.  See sketch of the slots size.  The walls of the button are 2mm.  

It's a small part, so you might need a brim.  Printed fine for me without one.