Viatron Computer Systems Corporation was a short-lived American computer manufacturer, founded in 1967 and defunct only four years later in 1971.
Viatron’s System 21 computers included a photoelectric encoder keyboard that may have been produced by Viatron themselves. The only vendors named are Autonetics who appear to have supplied the ROMs, and Tennessee Eastman who provided the keycap materials.
The schematics for the keyboard can be found under the /pdf/viatron section at Bitsavers. This keyboard uses four-of-N encoding at the optical level, converted to output codes by way of a ROM look-up table. The most likely reason for constant weight code encoding is for clash detection: if five or more lines are active at once, then two keys are depressed simultaneously and the keyboard must wait until this clash condition clears. The use of ROM look-up was state-of-the-art in 1969, which is the year that MOS/LSI-based encoding seems to have been taken up within the keyboard industry.
The keyboard also provides a means of tactile feedback that is also said to ensure that the transition from off to on at each photoreceptor is made as fast as possible, and can never sit in an intermediate position.
Detailed photographs of the Series 21 keyboard are available at Deskthority; the keyboard is said to weight 21 lb (9.5 kg), several times heavier than a modern notebook computer. The photos show perhaps the first time that the two shots of a double-shot keycap have been seen to completely separate; this is perhaps less likely with moulding techniques where the second shot is injected in between parts of the first shot. The keycaps are made from Tennessee Eastman Butyrate, which is a form of Tenite.