From my experience with 4-108 engines w/o inlet air heaters, they take the fuel
return line to the tank off of the top of the fuel filter
at the same fitting as the injector return fuel.
In your picture, they ran what would be the return fuel line forward to the vertical "manifold" and tapped into that manifold to feed fuel to the cold start "flame thrower" in the inlet manifold. So probably the threaded fitting that you are asking about is for a banjo bolt and banjo to connect your return fuel hose to.