It was indeed a previous owners plumbing of a return line. A hack job. . . hole looks like it was made with a file. Combination of washer, pipe fitting, nylon washer and a jam nut. I bet it was leaking / weeping since the day it was installed.
Car is currently is running a Holley HP950. No near term plans to need a return line. The hose you see in the picture was about 3' long and the end was plugged with a nut and hose clamp.
Attached is my slightly better, hack job. 1/2 bolt secured with SealAll. Cutting a new gasket for the sending unit, letting the SealAll dry and I will reinstall. Gave me a chance to make some minor adjustments to the float, in an attempt to get a more accurate read (fuel gauge reads past full when tank is full).
I think I will order a replacement sending unit in case my slightly better hack job does stay sealed. . . or for the next time I take the sending unit out.
I welcome further comments . . and I thank those who have commented already.
