The factory pickups only work on factory style pans only. Many aftermarket pans, due to the shape of the pan take their own pickup.
For example;
9337, Canton 15-389, and Milodon 30770 use the factory pickup
Canton 15-400, 15-444, and 15-452 takes the 15-401 pickup
Canton 15-450 takes the 15-451 pickup
Milodon 30355 takes the 18425 pickup
and so on.
Make sure you check the notes of each pan to see if it takes a specific oil pickup