I have this in my archived notes. They are Stallard ends, or similar. The prong is to keep the chain from derailing, according to the handwritten drawing. The text on the web that accompanies this drawing follows (from Vintage Lightweights):

"These drawings for Stallard rear dropouts as fitted to the Monlhery above are Percy's originals from the Stallard Archives held at Wolverhampton Library (see above).

The gear ends, lower left were designed for the Osgear. When setting up this gaer one needs to get the striker forks as close to the large sprocket as possible. With normal rear ends this creates a problem when removing the wheel as the sprockets jam up against the striker fork. With the Stallard rear end there is no forward travel of the wheel when secured in the forward position of the two. The alternative opening is for use with fixed wheel - probably in the winter. It is very difficult, if not impossible to use the Osgear rear ends for a Simplex gear.

The tang also stops the wheel from jumping forward when being replaced say after a puncture."

Now, where the wheel goes...hmmm.

Vintage Lightweights have several Readers Bikes fitted with Osgear and they are all set up like the OP's bike...in the forward part of the dropout. There are a few that don't run Stallard type dropouts, but these too have the wheel as far forward as it could possibly go...