| Which plate you are looking at (metal plate vs foil sticker) | 1940-1968 | 0 | Physical identification of which system applies. Foil stickers peel off and were often lost, in which case the amp is undateable without component cod |
| Model identification (Valco built the same chassis for many brands) | 1940s-1960s | 8 | Use the Supro listing as a guide for an unlisted Airline/National/Oahu/Gretsch version — the model NUMBER dates the chassis, the brand does not. |
| EIA component codes (the only witness once the plate is gone) | 1940s-1968 | 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 |