This thing has been bugging me a bit. Had a closer look today and there is another purpose that it might have. On the downstroke, the slot gives a larger opening between the inner part of the crank and the volume of gas that should now go up through the transfer channels. So instead of having just the small gap between crank web and the backplate for the gas in the inner part of the crank, there is now an opening pointing straight up in the crank web. This would then help to use more of the total crank volume, as the inner part of the crank is actually a rather large amount of the total crank volume in a racing engine with a large crank bore. A cambered leading edge of the crank web could then serve a similar purpose of not closing the volume at the crank web cutout.

I don't know if this is understandable, or even makes any sense, or not, but the bottom line is that the crank design could be mainly for the downstroke, when there is compression in the crank and gas should flow up through the transfer channels. As opposed to a special design for the upstroke as I had assumed before.