The 69 version has both a quicker ratio and a different 'spool' than the earlier versions. Up until 69 the 'spools' weren't variable. We started putting the variable spools in late 68. At the same time we designed a machine, a valve balancer, that balances the valve spool to give more assist at the times, in a turn, when it is needed. Since 69 all power steering boxes have gotten tested and 'balanced' right after assembly. I was 'captive' in this department for a few years and had to fix the balancers right quick because without them being balanced the assembly lines backed up. It was a 'HOT' job with lots of perks. All I did was sit back with a pager and wait for the call that a valve balancer was down. Lots of breaks and lots of glory when you get the balancer back up and running.