Due to a change in the oil delivery to the rocker shaft Chevy changed the rocker design. 1950 — 58 rockers had a groove centered in the in the hole for the shaft. On 1959 - 62 rockers the groove was offset ¼ inch to control the flow of oil. For an excellent explanation with pictures visit member Pre-68 Dave’s blog ( Chevy 235 guy). His site has an amazing amount of great information including the item listed as "235 rocker oiling". The information applies to the 261 as well.
