The transistor unit that replaced valves at Quad and became the anchor of thousands of systems.
Solid-state: check the output devices and the mains transformer tap; the 303 was built for several mains voltages.
Marks that can be entered for this maker: Serial number, Model / catalogue number, British capacitor code (Hunts, Mullard), Japanese capacitor code (Nichicon, Chemi-Con, ELNA), EIA component code, Version & era marker.
Nothing predates 1967; components only push the date later.
Sold-price aggregates are not collected yet, so the archive gives no figure — condition, originality of the transformers and a documented recap move this model more than any list price.
Evidence: Dada Electronics — Quad production history & serial numbers (PDF) (PDF)