In the early to mid 1960s, Seiko seems to have standardized its movement numbers to accommodate the many new design innovations, with the pattern xxxx-xxxx. The 3rd digit of the first set represents the grade of movement (so a 6246 is higher than a 6206) and the 4th digit represents the type of date display: 1 or 0 = no date, 5 = date only, 6 = day-date). The digits of the second set were meant to indicate design variations - but are difficult to interpret. Only general serials seem straight forward; the -8000 models are always dress watches with or without bracelet, the 5000s are always squares... The 7000s are somewhat dressy but also sporty...
Back to the Cronos, the aformentioned numbering transition seems to have occurred around 1962-63. Many of the pre-1960s (3-digit) movements remained in service but were simply given new movement numbers with 4-digits.
Seiko's major design innovations in the sixties came out of the Suwa factor. This was the 'Marvel' model which was introduced in 1956. The innovation of this product was the addition of an auto-wind capability. Seiko called this capability the 'Gyro Marvel' and gave it a movement number! This model was known as the 290 and it was produced from 1956-59 by Suwa. It was finished with 17, 19 or 21 jewels - at different price points.
The Daini design team soon followed with the 'Cronos' model in 1958 - and named the movement '54A caliber'. This movement was thinner (a Daini theme) and had a bridge anchored at both ends, using the Diashock technique. Daini designed and manufactured high-end manual watches from 1958 to 1964 in 17, 21 and 23j versions. These were also available as the Cronos Selfdater (with date) from 1961 and the Cronos Special (with extra adjustment) from 1963. The Cronos was Seiko's mainstream offering beginning in the late 50s, similar to the Marvel and was priced from JPY 5700. The Cronos watches were generally precursors to King Seiko and Grand Seiko though not quite as high-end. A refinement of the Cronos movement was later used in the King Seiko.
Suwa quickly responded to the Cronos in 1959 with the 'Crown' model, using a new movement, the '560 caliber'. From this base model, Suwa also developed the 3180 Grand Seiko in 1960. The 5720 Grand Seiko also came out in 1960 with 25 jewels, followed by the 5722 calendar at 35 jewels in 1963. All these movements were hand-wind-only.
Both factories were very busy improving their movements and competing with each other as well as with other brands. The Crown and Cronos went through several versions. In 1961 Suwa introduced the 57A and B versions of the Crown as well as the 5760 Special and the 341 Special. Each of these releases was an incremental improvement of the Crown design and were created with different levels of finish with different price points. This allowed customers to select from a range of different qualities and feature sets. Simultaneously, Daini developed and released the 810 Cronos Special in 1962.
The culmination of these efforts, as stated, were the King Seiko and Grand Seiko movements of these two divisions. Especially the Daini King Seiko 4522 and the Suwa Grand Seiko 5720 were very close competitors for the horological honors. They both however came to a full stop during the ascent of quartz...