The slot is covered by the frame with the rear wheel installed and is normally inaccessible. I would suggest that the backing plate slot is located too high on the plate centerline to be any sort of effective drain; the brake would literally be swimming, well before water could reach the level of the slot. The M20 brakes are in no way sealed to begin with, so I wouldn’t imagine that water retention to that level would be an issue.
An opening for “ pressure equalization” seems a stretch, still, I’ve no better ideas…!
Always have thought it is grease "outlet". Some earlier? rearplates have this flange added:
But cannot see how grease from the hub will reach the brakeplate, but the spindle, drum and hub has had a few redesigns in the late 30s
so maybe the hole is a leftover from an earlier design?
The frontbrakeplate also has a design to catch grease from the hub:
It is a leftover from the early speedodrive but think it still has a function.
Your comments make sense, in an earlier design the slot provides an outlet for any excessive lubricant being thrown out of the open wheel hub bearings.
I’m familiar with the front brake backing plate grease “trap”, however I have never seen an earlier rear plate complete with the, apparently discontinued, sheet metal baffle. The baffle was dropped, but the slotted backing plate continued on even though the slot is now useless for it’s originally intended purpose.