| Model series and re-numbering eras (M-xxx -> 100 series -> 200 series) | 1946-1967 | 14 | Era / model-run dating only. Two amps of the same model in different years are indistinguishable by model number; component codes (speakers, caps) mus |
| Covering and cabinet shape as an era marker | 1946-1962 | 4 | The history states a clean split: no dark brown Magna Electronics amp had tremolo, no light brown amp had vibrato, and no MOTS amp had either — a usef |
| EIA component codes (speakers, capacitors, transformers) | 1940s-1960s | 2 | 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 |