https://www.beyondthedial.com/post/seiko-bell-matic-the-complete-guide/

Seiko’s Alarm Watch

In 1966, Seiko developed its own alarm watch, the BellMatic, containing either the 4005 or 4006 movement. The 4005 movement is significantly rarer than the 4006 since it was only used in 1968 and 1969. As is normal with Seiko movement numbers, a caliber with a number ending in 5 is the date only variety, while a caliber number ending in 6 is the day/date variety. Other than this difference, the movements are the same and we generally refer to the Bell-Matic movement as 400x when discussing both.

The ‘bell’ in the Bell-Matic is actually a curved metal sounding spring that is repeatedly struck by a small hammer powered by a spring unwinding. That spring is released by a lever moved by a wheel sitting on top of the hour wheel on the dial side.

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The 603x models are a transitional design between the early-70s UFO-cased, colourful sports diver-esque Bell-Matics and the tight, efficient cases of the run-out models of the Bell-Matic line. The stainless steel or gold squared-off cushion case is vertically brushed but otherwise unadorned and fuss-free. Dials for the steel watch are either sunburst blue or silver and the gold watch came with a plain brown dial. In each case the colour of the indicator ring matches the dial.