It is a BMC A series engine. Most commonly used in the Austin Mini.
This one is likly from a rear drive application such as an Austin Healy as it does not have the sump mounted gear box.
BMC owned a plant in Belgium that made cars and engines.
The OD green is a factory painted colour on these engines.
More info on the A series here
http://en.wikipedia.org/wiki/BMC_A-Series_engine
and here
http://www.aronline.co.uk/index.htm?engineaseriesf.htm
Hope this helps
Matthew