I think the last digit refers to cylinders, 3406 is 6 cyl, 3412 is 12 and the discontinued 3208 was 8.

The C7, C9 and C18's are 6 cyl.

The old Detroits indicated displacement, 8/71 is 8 cyl, 71 c.i. per cyl and a piston from an 8/71 would work in a 2/71 or a 16/71, the cat stuff is a mystery and although I've owned a few, I've yet to figure it out!