It's actually '71 and earlier use the staggered bolt. Starting in the '72 model they were symmetrical. That part doesn't matter all that much as the one offset hole in the pulley can be filed to accept either balancer. The timing marks changed when they went from the cast iron water pump to the aluminum type. The timing cover changed then too when the timing marks went from the right side to the left side (passenger side and driver's side on LH drive cars). There were also a few different balancer appearances. Some were like the OP's that have a flat forward surface on the outer ring along with a semi-hollowed slot in the ring on the timing cover side. There are also versions that are flat on both sides. So, it looks to me like the OP needs a '70-'71 318/340 balancer if he wants an exact replacement. If he can live with slotting the crank pulley, he can used the '72-up version. If he wants to cut a new ignition timing slot in the outer ring, he can use the '69 and earlier balancer. There are also businesses that can rebuild your balancer if you want to keep the original part.
