Simpler is always better-since I am using an AC source, why rectify it and create headaches needing a diode and resistor to mitigate? Why not just a 120v to 12 v transformer and a relay with a 12 VAC coil?
BTW, the two black boxes are 2 pole disconnect switches. Given these examples as well as Frank's switch above it seems it was common to switch both legs.
Of course, if you read through the original version of the NEC (available on Mike Holt's site), the concept of neutral apparently was not yet established.