| Seven-digit serial formula (year digit + month + sequence) | 1970s | 2 | The year digit is a single digit, so the DECADE is not encoded — 1 could be 1971 or 1981. The decade must be fixed from the model era / cosmetics / co |
| EIA codes on filter capacitors and circuit breakers | 1960s-1970s | 3 | Component codes date the PART, not the amp: a replaced transformer/speaker/pot can be newer than the amp. Upper-bound rule — the amp is not older than |
| Dates on schematics and QC paperwork inside the amp | 1960s-1970s | 0 | Dates the drawing/paperwork, which brackets but does not equal the build date; use as corroboration. |
| Model era table (YBA / YGM / Mark II / Mark III and modern lines) | 1960s-present | 8 | Era dating only. |