There's actually a couple different designs for the rocker shaft oiling. The 56 and earlier had drain back port. It was common practice to crimp in shut. It was a thought that it helped the outer rockers get more oil, and it helped with engine oil pressure.
The 57 and newer blocks didn't have the drain back port. That was due to changes in the oiling system They had an orifice opened up on the block.
I have 2 235's in my garage right now. The 55 block has the drain back. Whereas the 59 doesn't.
6 cyl. oil flow More good info.
tech tips 
