Peter,
I have had a look through the various bits of stationary engine information I have and come across the following. Your engine in question would appear to be a VS25 250cc sv manufactured during the second WW. About 1945/46, Douglas published a leaflet on their Industrial Power Units. It stated that
" It is now possible to give preliminary details of the range of Douglas Engines which will be available for peace time applications, and you are invited to consider the incorporation of a Douglas Power Unit in any post war project for which an i.c. engine up to 1000cc would be suitable. On the following pages are given abridged specifications of the engines which are at present, or which will shortly be, available. Other types are now being designed and developed on the same basic principles, and if your requirements are not fully met by any of the engines listed, please do not hesitate to write. The needs of war have demanded that many exacting conditions should be fulfilled, and the unrivalled experience of Douglas technicians is placed at your disposal.
Applications.
It is impossible to give a complete list of all suitable applications, but the following examples will serve to illustrate the versatility of application of the Douglas engine - Generators, Battery Charging Sets, Pumps, Agricultural Plant, Industrial Power Trucks, Marine uses (small craft etc), Builders and Contractors Plant. "
The leaflet then goes on to give specifications of six h.o. engines ranging in size from the 350cc with three engines of 804cc and two of 995cc. Four vertical single cylinder (side valve) engines were also listed comprising a 250cc (VS25), 463cc (VS45), 543cc (VS55) and a 982cc (VS98)
The VS25 had a bore of 65mm and stroke of 75mm. A length of 14", width of 19" and height of 20"
I think it can be fairly assumed the majority of these engines failed to materialise as manufactured products. It may be the single cylinder engines were developed for the post war 450cc, 540cc and 630cc engines that made production until the early 1950's. The 350cc flat twin engine was the only o.h.v. h.o. engine produced with the others of 804cc and 995cc being side valve engines with a choice of air cooling and water cooling. As we know the 350 with the FT35 engine formed the basis of the first post war Douglas motorcycle the T35 or Mark 1 as it became known. It would be interesting to know if any of these larger side valve engines both single and twin were in fact manufactured and if so, how many survived. If Peter is interested, I can supply a copy of a General Arrangement drawing of the 250cc industrial engine.
Reg
