I wonder if the hole diameter differs? Angle of the tip of the needle?
The 396521 seems to cover the small hp motors as well, while the 396520 seems to start at about 90hp motors. Mainly newer motors as well. Well, for us anyways.
Of course just speculation. I really have no idea.
I had hundreds upon hundreds of nos needle/seat sets. I separated them out to 2 buckets, 1 for small hp motors, for large hp motors. I mainly do smaller hp motors, so that bucket is getting low. Im going to try the one for the larger hp motors and see what happens once I run out.