Kubota 16271-43930 is the spring for the fuel filter assembly (part number 16271-43561) in the Model D902 diesel engine. Detail 16271-43930 is installed in the filter cup as shown in the Illustrated Part List from Kubota manufacturer (see below).
Shindaiwa DGW400 Diesel Welders are equipped with D902 engines, which is the main mechanism of the electric generator in the unit. Kubota 16271-43930 is a small size spare part and can be lost during maintenance and filter replacement. Due to the latter, it is recommended to have one more a spare part when replacing the fuel filter.