From CRG numbers/ decode section: "For cast-in date codes on GM-produced iron and aluminum castings (engine blocks, heads, intakes, water pumps, etc.), the letter "I" was used for September, and the last month of the calendar year (December) was the letter "L". For stamped-in date codes on GM-produced components (alternators, distributors, starters, etc.), the letter "I" was not used - "I" was skipped, and "J" became September, with December being "M"."