The 1903 lineup for Royal Enfield motorcycles included three models. The engine clamped on the front tube is finally gone. One of the machines has the engine behind the front wheel clamped just in front of the bottom bracket of the frame. No pictures seem to have survived of these Minerva 2hp sidevalve machines but they must have resembled the Minerva motorcycles, The other two have the engine underneath the rider, like modern motorcycles. Apparently this idea was copied from Werner, who had a patent on it. To circumvent the patent, Royal Enfield did not include the engine as a stressed member of the chassis, as Werner did, but designed a frame that looped around the engine.
The first of the other two machines was quite advanced, it was water cooled, had chain transmission driven by a complicated mechanism involving the camshaft. The water was kept in a tank in front of the petrol tank and had a small radiator for cooling. Oil was kept in a compartment of the crankcase. More details here. It had a 277cc 2.25hp engine and was quite expensive at 75 Guineas.
The third machine was more economical, being air cooled and belt driven, with a 239cc Enfield made engine. It costed 45 Guineas. No photos seem to be available of this machine in 1903 trim, though we will encounter some in 1904.
At the Stanley show, in addition to the three motorcles, two models of the quadricycle and one of the tricycle were also shown. They were offered with 2.75 or 3.5hp De Dion engines and a Bozier two speed gear. The quads were essentially similar to the 1902 models. The trike had a lengthened frame, 2.75hp engine but single ratio direct gearing to the axle.
November 1903 saw the establishment of an entirely separate factory at Hunt End for car and motor cycle manufacture. In addition to the motorcycles the company was selling two car models, a 6hp single cylinder voiturette with a De Dion engine and a 10hp twin-cylinder vehicle powered by a side-valve Enfield engine, with a three speed gearbox and a four seat body.
(Source: "The story of Royal Enfield motorcycles" by Peter Hartley, Patrick Stephens Ltd., Cambridge, 1981)