There were indeed service replacement 427 side oiler blocks. I've seen more than my share. They have the ribs. They have the cross bolts. They have the side
oil galley "bulge" and they have the front galley plug on the driver's side of the timing cover.
No, they weren't in 60's OEM vehicles, but they were legit, and were just offered over the counter as replacements. (You could also get Boss 302 service replacement blocks, after the 69/70 Boss 302 Mustangs were made....)