On the 351C-2V engine it screwed into the fuel inlet on the Autolite 2100 carb. Don’t know about the Autolite 4100 4V, never had one. On my 351C-4V with a retro fit Holley it’s an inline just in front of the carb. I wasn’t to thrilled with the clear plastic body on the aftermarket fuel filter but it was great at seeing that I had a small air leak on the inlet to the fuel pump that was causing the engine to die on hard acceleration (entry ramps to highways). It also showed all the rust coming from the old gas tank and made it clear that I should replace it.