Marantz is the clearest case in this archive of a popular brand whose numbers are only partly readable. Three different things can be dated, and four numbering systems cannot be dated at all — and the important part of the work is keeping those two groups apart.
What is documented
Official years exist for the tube icons, from Marantz itself: Model 1 — 1954, Model 7 — 1958, Model 9 — 1960, Model 10B — 1964. A handful of receivers are documented from a compiled sold-price archive: 2230 (1972–1974), 2270 (1971–1975), 2245 (1971–1976), 2220 (1972–1974), 2010 (1972–1973), 2325 (1974), 2500 (1977–1979), 2600 (1978).
The chassis stamp
On a Marantz receiver the strongest date evidence is often not the serial number at all but the stamp on the chassis. The archive holds the documented form 77 9 8, read as 1977. The source does not state the full meaning of the trailing digits, so neither does the archive: the year is reported, the rest is declared unread. The stamp can be hidden by dirt or by a rebuild, which is exactly why it is worth photographing before anything is cleaned.
The two-digit prefix, and who believes it
A second system reads a two-digit prefix as year plus a week period. The archive flags this as the compiler's interpretation — the source's own wording is "we believe" — not a Marantz publication. The same system supplies the production-run form (93 N 04 0085: run 04, unit 0085 within it) and the region letters: A Australia, C Canada, E Europe, K Far East, N unknown (usually Europe), P Post Exchange, U USA. The region letter is not a date, and a European unit can look "later" than a US unit of the same era because some European serials started at 50001.
The four systems with no year
Four Marantz numbering series in this archive carry no year on any row: the sequential Model 1 numbers, the M2 production-run series, the regional starts, and the 2235 / 2250B pair. They are published as undated, with the reason, rather than filled in by analogy. A number that runs from 1001 to 58,000 without a documented calendar says nothing about when the unit was built — and saying that is the honest answer, not a failure.
Where to go next
The Marantz route in practice: identify the model, check the chassis stamp, then read the component codes on the transformers and the British capacitors if the unit was assembled in Europe. Start at the Marantz archive, or read how chassis and platter stamps are read.