They changed the way the oil gets up to the rockers I think it was 1958.
On earlyer engines you have a pipe going thru the side cover. Later engines used different lifters (with an anulus)and the oil comes up via a passage in te block.
If you have a later engine and head, the rockers are different. They have a offset oil groove at the bushing in opposit to a centered one on the earlyer models.
If you use the older rockers (with the centered groove or anulus)they flow much more oil and you get a big oil mess out of the valve cover.
SO...
if you use a post 58 block and head with anulus lifters an the oil line in the side cover you need the offset anulus rockers. Also you don`t want to have the overflow pipe between the rockers.
if you use a pre 58 block and head with the oil line and the straight tall tulip style lifters, you need rockerarms with the centered oil groove and a overflow pipe between the shafts
if you got a -lets say- 59 block and a 55 head GM service shops used a hollow head bolt for this combination - and it is getting a bit complicated.
hope that helps, Frank
