I've dredged up a little info on early vintage Rovers. I know I have a potted history somewhere but finding it is a different story.
Obviously Rovers made both cars and bikes in the early days and the model shown is the last model bike they made before concentrating on cars. Some of this info I remember from the short history I have and some I am quoting from the Olympia Show Report in "MotorCycling" of November 1924 reporting on the upcoming 1925 models.
The bike was originally a 250ohv and very modern for its day but as usual riders wanted more power so for 1925 Rover upped the engine to a 350. It was of unit construction with the engine and gearbox in the one unit like todays bikes and the valve gear was totally enclosed. It had a modern lubricating system with the oil be circulated with a submerged oil pump in the sump. Oil capacity was one quart (bit over a litre). Not really enough and I think this was its Achilles Heel. As you can see from the photo it also had an enclosed primary chain. This was all about ten years before these things became pretty well standard practice in the rest of the motorcycle world. A very interesting and smart bike and there are a few survivors around.