ESO was a Czechoslovakian motorcycle manufacturer. The Czechs built their own motors and the ESO was considered to be near bullet-proof. The ESO Model 250 was a 250cc four stroke that was as big and heavy as the open class bikes. It sported a 249.5 cc engine with a bore of 70mm and a stroke of 64.8 mm.
A side float carb and a high straight pipe were considered high-tech at that time.