I'm not sure I see a computer.

Your just not looking hard enough. The whole circuit is a computer.

When the label says "Computing", everyone assumes that there must be a micro controller or micro computer in the design. I guess that's a sign of the time. But long before they computed digitally, they computed with analog circuits. With analog, its fairly simple to do addition and subtraction. But when your trying to do multiplication and division, there is a problem. That is, until you convert the linear voltages to logarithmic voltages.

The Op-Amps (LM348) have much the same characteristics as the old uA741. They are just lower current and have smaller offsets. And there are four of them in the package. The diodes (1N4004) in the Op-Amp feedback, take care of the log conversion. After that it's just adding/subtracting the corrected values. Now, I know you know the equations necessary to calculate VSWR from the Forward and Reverse voltages.