Collins KWM2-A "Vietnam mod" Why is it that way?

[neazoi]

First page of this PDF describes a way to stabilize the voltage to the VFO of the Collins KWM2-A, so that it is not affected by the lowering of the voltage during transmit from the otherwise unregulated PSU.
First picture shows the original circuit (just a voltage divider) and second picture the modified circuit.

I wonder why should I replace R73 to a higher value? Can I just leave it there and place the zener in place of the R131?

Also why should the 470R exist? The only thing it can do is lower the voltage further lower than 150v. Is it there for some kind of protection?

 

[G4BCH]

Also why should the 470R exist?

It tells you in the last paragraph. I guess that it helps to reduce the zener noise

I wonder why should I replace R73 to a higher value?

The zener does not need so much shunt current, also reduces dissipation.