This little piece of history recently appeared on our local auction site, it was unusual because of it’s near-new condition, the feature-set it supports given its age and having not only the original manual, but also the carry case and original box. It is a Colex model ESR, which is given to stand for Electronic Slide Rule. The last image below has the full specification.

While it says ‘Made in Hong Kong’ and the serial number is E65935, there is no indication of the date of manufacture. According to www.eepcworld.de a model with the serial number B58599 has a date of 1975, which I assume to be the date of manufacture (since I am mono-lingual and the site is in German).

The calculator can be run off 4 x AA batteries or an external 6 volt power supply. The polarity is not specified, but one assumes it is DC. The current draw is 150 ma. (Ouch on batteries). The display is green fluorescent, which from memory requires an internal inverter to generate the required high voltage. The calculator is in perfect working order.