Horizontal mount Wico magneto on the front of the engine where the generator would be on a WL.  Magnetos were 'stock' on WRs but due to the small production numbers it wasn't worth them developing their own so they adapted tractor magnetos.  The early Wico caps had four spark plug lead sockets with two blocked off, hence the diagonal arrangement on this one.
 The very early WRs had a horizontal mount mag with a WL-style cam cover minus the provision for a vertical mount mag.  The difficulty came in timing/adjusting the mag so it later moved to the classic vertical position.  Then in 1950 they redesigned the cam cover for horizontal mags with a removable cover to aid mag timing/retard/advance, which is what this bike has.
Not quite sure why HD were so keen on the horizontal position.  I guess it uses a more robust gear and is solidly mounted out the way.