Hi All
I have two sets of #18 heads that have different rocker rails on them. Not sure if this is factory or someone swapped parts from a different engine that are incorrect. What I've been able to find is the heads with the code stamp starting with a C have rails that go all the way to the valve cover and have a support under the end. Then a set with the code stamping starting with a D has shorter rails that do not go all the way to the valve cover and do not have a support under the ends of the rails. The ones with a letter C came off a 1959 371. Then the ones with a letter D stamp I got in a parts collection. I did get a 4bbl intake and carb wtih a tag on it in case that might help key out the application.

Photos of the C code stamped in it.

 

Then photos of the second set of heads starting with the letter D stamped into it.

 

Photos of the intake I got with the D stamped heads. I see now the carburetor is a Carter so it may not be original. I thought this vintage was using Rochester carbs.

 

The first head is indeed a 1959 head, as evidenced by the "C" prefix on the engine number.
The second head is a 1960 head, as evidence by the "D" prefix.
According to the 1960 Oldsmobile parts book, there was a change that eliminated the rocker shaft end brackets.
This change was made after the first 12,000 engines.
The manifold, according to the casting number , is either a 1959 or 1960 four barrel manifold.
The Carter carb is probably from a 1952 thru 1954 Oldsmobile.
Olds didn't use any Carter carbs after 1954.
I cant' read the number on the carb tag , or I could tell you what year it was.
1952 = 932S
1953 = 2016S or 2080S
1954 = 2059S