Did I do something stupid?

I will recount to make sure and look at the parts numbers.
Did you recount the number of leafs at the perch? If the pictures on Summit are correct, then they both have 5 leafs. The bottom leaf on each spring looks kind of like a half leaf. The difference being that "half" leaf points forward on the 863 spring and points rearward on the 864 spring. That can be deceiving when counting leafs as I said earlier. My P3412002 (right) and P3412003 (left) have that same design. The right spring (002) has that half leaf pointing forward same as the 863 picture I see. So I am still thinking your 863 spring goes on the right side as you have it. Take a look and tell us what you see.