If at some time in the past a earlier top end was installed the mechanic could have installed a plug in the block oil port and used a special drilled head bolt as per chev parts book instructions. Here is what the parts book says.
Drilled 235-261 Head bolt:
From 1938-62, only two head bolt part numbers are listed for 216, 235 and 261. Long and short except for a special drilled bolt, used when a 59-62 block is used with earlier top end* to reduce the oil flow to the rockers.
Note: Use with drilled bolt
Plug # 37 64 579
Install in block oil port near right center head bolt.
(This plug blocks the original oil path and sends it through the drilled head bolt)
* The cylinder head did not change but the rockers did.
