1951 F5. I have the original style engine oil filter, but its missing a oil line. Only the lower line that goes down into the left side of the engine is there. Where does the other line that mounts to the top of the filter assembly go? Why the heck was it removed? I assume it goes somewhere in the rear of the motor? No idea?

 

It comes from a port on the back of the block, just in front of the bellhousing. I'd bet your oil pressure sender is screwed into that port now. Normally when the optional filter was installed, they put a Tee at the filter end of the line, with the sender in the branch of the Tee. The connection to the filter includes an orifice that limits flow to the filter (because it robs some oil pressure even with the orifice).

I'd guess the line was removed because either it cracked and was leaking, or in an attempt to boost oil pressure on a worn engine.