The curve goes toward the front. Or you can reinstall it with the curve facing to the rear, loop a chain over the axle, and then tow your truck around a bit being sure to jerk it hard a few times checking each time to see if you've straithened it back out.
My bet is that curve is the result of someone towing the truck with a chain over the axle and jerking it a few times. Not a good idea.
I don't know how hard these axles are to find in Europe, but if it were me I'd go shoping for another one.
