Curiouser and curiouser!
The eyelets on the rear dropouts are more likely for mudguards, not a derailleur.
The vertical dropouts are pretty unusual. This bike was made for a derailleur that didn't mount on the dropout. Judging by the double cable stop, it was a derailleur that used two cables. Nivex, perhaps? How did the Simplex 543 mount?
I like the Ephgrave theory.
rhm, the double cable stops are on the downtube, it looks like only single cable stops behind the bottom bracket. I believe some Osgears had a tension release cable for the front arm and the cable stop setup on the OP's frame would fit this arrangement.
I also think MauriceMoss may be spot-on with Ephgrave.