that's a oil feed hole, not return.
Oil is fed thru that, thru center stand to the rocker shafts were is then feeds the rockers themselves.
It's common on a rebuilt to use .090 restrictors to limit the amount of oil to the heads.
http://www.fordforums.com/f500/390-oil-restrictors-160176/
http://www.carcraft.com/techarticles/ccrp_0808_ford_390_fe/pictures.html