[Chris Marshall]

I believe Mart has it correct. The early flatheads (pre 59) that used the slinger for oil control on the rear main had the tube in the main cap for oil return. The tube was no longer used from the 59 blocks on when Ford changed to the rope seal on the rear main. I have a 99 block with the tube hole in the main cap and a 59L block where the rear main does not have the tube hole.