David,
Oops, looks like I need to eat my words.
Some of the engines did have a timed breather in conjunction with that arc shaped slot. Such as the 1925 250cc models (see picture of 250cc crankshaft).
But it has been so long ago, I could not remember if the 600 Aero has the rotary valve or is it was omitted; and mistakenly assumed it did because of the slot. I just whipped the timing cover off mine, and see no rotary valve. Also the extra breather hole is present, 40mm off the floor of the crank case chamber, just like yours. Unfortunately the angle is too acute to get a photograph.
So either the hole is to supplement the breathing through the curved slot, or supplant it. If they used a oil thrower on the timing side, then that suggests they wanted to supplant it. Why, I have no idea. Normally I would expect a thrower just on the drive side to make life easier for the felt seal. It was standard design with the Dougies for the oil to end up in the timing chest eventually, so why block its path? I see what you mean in the 1938 parts list, an oil thrower is clearly mentioned for the timing and flywheel end of the crankshaft.
-Doug