My factory original intake is dated L057 on my 1/29/68 Bird. I suspect that means December 5, 1967 (about 7 weeks lead).
What is confusing for the end of year parts is some GM parts seem to use "I" for September and some skip it; meaning December is "L" for some parts (e.g. engine parts castings) and "M" is December for others (e.g. alternators).
