Stevens's Model Dockyard were established in 1843 (according to their catalogue) and were in existence for around 100 years. This model appears in the 1919 catalogue and also (unchanged) in 1931. It was probably available a few years before and after those dates. It was available in 4 sizes and this example is size 4 (1" bore x 1" stroke).
The catalogue entry reads "Slide valve cylinders with rectangular ports, screwed packing glands, trunk guide and crosshead, steel connecting rod, bearings, heavy flywheel, 3-speed pulley, brass bedplate, made of brass and steel castings, all parts screwed together. Highly finished, fast running, guaranteed British make and British Materials, the whole mounted on a plinth and good value. Will work a dynamo or other models." The price in 1919 was 45/-, which was quite a lot in those days.The wooden plinth is not original and has a cut away section to clear the flywheel. It is unpainted, which is, I assume, how it was originally.