| Serial-number prefix and location by model | 1969-present | 59 | Identification, not dating: the prefix tells you the MODEL, and only inside a model does the number have sequence meaning. Use it together with the bo |
| Handwritten build date inside the chassis ('birthday') / tech initials | pre-1990 (handwritten), 2008-2009 onward (initials + date next to the serial decal) | 4 | This is the only truly authoritative date for a Mesa and it is NOT derivable from the serial — it must be read off the amp. On some models it is absen |
| EIA source-date codes on components | all eras | 0 | 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 |