From what I understand, 317 indicates the casting number of an unassembled head, while 035 indicates an assembled head. Why they did this is beyond me, but when I got my new heads and saw 035 I went on a hunt to see what the hell they were and this is what I came up with. On the 317 heads I had, they did not have the semi-perm cast marks as the 035's do. Maybe the different casting plants just use different numbers.