There were 1648 sold as 500s. Apparently all of the 850s sold in Europe were diesels. Here's what it said in the article: "All David Brown 850s were diesel models. Crop Master gasoline engines were used in the rebadged tractors exported to the U.S., where gasoline tractors remained popular.".
Along with this quote. An article in Hart Parr Oliver Collector in 2006 gave the same figures. "As for the deal between Oliver and David Brown, which started in 1960 and ended in 1963, 1,648 Oliver 500 models were produced, more diesels than gasoline models (all of the 1963 models were diesels)."
Read more: http://www.farmcollector.com/tractors/reviving-an-oliver-500.aspx#ixzz2ysO2ss5C
Read more: http://www.farmcollector.com/tractors/reviving-an-oliver-500.aspx#ixzz2ysNB8Gaf