At http://musicfromouterspace.com/analogsynth_new/WALLWARTSUPPLY/WALLWARTSUPPLY.php, Ray Wilson wrote:
Diodes CR3, CR4 and CR5, CR6 are protection diodes recommended in National Semiconductor Application Note AN-182 for protecting three terminal regulators from unwanted current paths that the output capacitors can inadvertently cause under certain conditions.
So I took a look at AN-182 (this version’s revised 2013, after Wilson wrote the above apparently). It does talk about two diodes per regulator, but one is to block current from a capacitor connected between ground and the regulator ground pin, which doesn’t look like it’s relevant for the MFOS circuit.
But there is also this:
When a positive regulator (except for the LM117) is loaded to a negative supply, the problem of start-up can be doubly bad. First, there is the problem of the safe-area protection as mentioned earlier. Secondly, the internal circuitry cannot supply much output current when the output pin is driven more negative than the ground pin of the regulator. Even with low input voltages, some positive regulators will not start when loaded by 50 mA to a negative supply. Clamping the output to ground with a germanium or Schottky diode usually solves this problem.
Note this refers only to positive regulators. “Negative regulators, because of different internal circuitry, do not suffer from this problem.”
Anyway, I was looking for information on whether that diode needs to be a rectifier like 1N4004, and instead found something that specifically recommends a Schottky. So that’s good.