So, I found a clue to the reason behind the slots in the bottom bracket on the Classic Lightweights site, in the "Block Chain" essay:
An interesting solution to this problem is evident in the design of the Harris/Plattner Raleigh seen in the image below.
Under its bottom bracket is a chiselled slot which allows easy access to the five pin fixing bolts attaching the ring to the crank.
So they are there to let the bolts of a 50.4 crank be backed out without having to take the drive-side assembly off the spindle.