SDH,
Yes, It wasn't clear. The second six pin connector was pushed into the slats of the front fan assembly:
>and I assumed that the cable was the front fan power connection.
And both 600W z420's have two six pin connectors. But, I see that the OP's z420 is a V1 and does not have the front fan.
You're correct that the 400W power supply does not have an integral 6 pin. The 400W system had a single 6 pin, connected to the Molex 4-pin, but that may have been an added for an optional GPU or by the original user. Z420_3 was the only z420 purchased used.
While current GPU's are very efficient for power useage:
https://www.studio1productions.com/Articles/NVidia-GPU-Chart.htm
> it seems any workstation with two GPU slots would have some provision for supplemental power.
The evolution of GPU power efficiency is striking. Note in the chart that a GTX 480 and Titan XP both require a 600W power supply and a GTX 550 and GTX 1060 will run on a 400W.
BambiBoomZ