Hi Steve - that's a very nice brochure that you have. To answer your question, these single cylinder Douglas engines were definitely produced in significant numbers post war. They were available in 450, 540 and 630cc versions, with several different configurations. most were straight petrol, some petrol/kero (TVO) and some with gear reduction drive. They were usually sold to manufacturers in batches (Trusty Tractors is one and here in southern Australia, a number were sold to a Company named MORTON who fitted them to swingsaws). If you go to website
www.duster.org.uk, you will find out more about these. I have collected several of these Douglas engines over the years. They are 'over engineered' extremely robust, very heavy, but a very reliable stationary engine. Hope I have been of some help. Regards Nev