From 1936 through the sixties, reading Zenith model years from a model number is as easy as reading the model numbers themselves. This same mechanism was used in chassis numbers from 1941 to 1949.
For model numbers the key is the first numerical digit following the chassis type letter (see letter codes).
In 1936, only two digits followed the letter model type so the lack of a third digit indicates this year. e.g.7 S 28
For 1937-1940, the year is only in the model number, chassis numbers were simply assigned in sequence and carried forward across years.
NOTE: Where model numbers are concerned, Zenith often kept the same model number if a unit was continued without major change for several years. Hence the model H-500/chassis 5H40 was introduced in 1951 (H model year) and was continued into 1954 as the H-500 even though several chassis changes occured. Thus Zenith was one of the first to "slipstream" in fixes during the run of a model.
1937 - 1 e.g. 6 S 159 1938 - 2 1939 - 3 ` 1940 - 4 In 1941, chassis numbers also gained a code but in a letter . Year Model code Chassis code 1941 5 (12 S 595) A (12 A 4) 1942 6 B (no civilian production Apr 1942-1945) 1946 0 C 1947 7 D 1948 8 E 1949 9 F
In 1950, things changed again with model numbers adopting the same letter code the chassis used except it became the first digit in the model number. Note that the second digit in the model number is indicative of the number of tubes (later transistors) while the first digit in the chassis means the same (where chassis and model numbers differ - usually by one - trust the chassis number.
1950 G (G-500) G (5 G 40) 1951 H 1952 J (skipped I) 1953 K
1954 began with "L" but with proliferating models, two digits were insufficient so more letters were used. Some were skipped (O,P,Q) to avoid confusion with other models.
Just to really confuse matters, when Zenith began producing transistor radios they received an extra letter near the front - a "T" so that in 1957, the six "tube" Trans-Oceanic was model/chassis A-600/6A40, while the nine transistor "Royal 1000" was model/chassis 1000/9AT40. Transistor models ("Royal"s) lost the active device indicator in the model number (became pure marketing) but could still be tracked by the chassis number.
1954 L, M 1955 R,S 1956 T,U,W 1957 Y,Z 1958 A (ran out of letters but first of new system was G so...) 1959 B 1960 C
(There is indication that this continued erratically until 1982 when "M" was used for the third time. "SWAG" is that H was used in 1962 , K in 1963, Z in 1968, D in 1972, K in 1979,and M in 1981