Heuer. A stainless steel automatic calendar wristwatch Model: Calculator Date: Circa 1977 Movement: Gilt 17-jewel Cal.15 automatic Dial: Black, applied luminous filled polished baton hour markers, white outer 1/5th second divisions with orange five minute markers, subsidiary dials at 3 and 9 for 30 minute and 12 hour recording, date aperture at 6, orange pointed baton hands with luminous inserts, centre square tipped chronograph hand Case: Brushed and polished cushion form, screw down back, large rotating bezel marked for calculations, crown at 9, twin chronograph pushers at 2 and 4 Strap/Bracelet: Black leather Buckle/Clasp: Associated steel buckle Signed: Case, dial & movement Size: 45mmm Footnotes: Before the arrival of the integrated circuit and the computer, the slide rule represented quite the pinnacle of technology. Allowing the user to perform complex calculations quickly and being able to use one was quite a skill; for working out anything from the service to pay at a restaurant to the amount of fuel and range remaining for an aircraft. Although Breitling were probably the most well known for producing slide rule equipped watches, the Calculator joined the Heuer range in 1972 and was powered by both the calibre 12 and 15 movements during its production run.