Strange???
The Douglas patent 21259 of September 1913 (Improvements in the method of attaching speed change boxes to engines) shows a gearbox of the 4HP type clamped on a dovetail slide to the back of the engine. This gearbox carries that patent number, but is clearly of the 2¾HP type that bolts up under the frame. The only difference appears to be the extra lugs fore and aft to carry the clutch/kickstart mechanism (similar to the 1912 models fitted with a clutch). Another oddity is that it has the remains of what looks like a 4HP type clutch on the primary side rather than the normal cone clutch in the belt pulley.
Eddie.