The first letter represents the year in a 10-year cycle (e.g., H=1, I=2, J=3... up to S=0).
It is the primary source of truth. It explains the "Internal Dating System" (used for exports) versus the "Unit Number System."
The absolute "best" place to start is the source. Yamaha provides an official digital archive (often a PDF or a web-based database) that covers almost every system they’ve used since 1966.