Seiko Slide Rule aka Calculator. A pilot watch. Please search internet to see how to make calculations with its bezel and slide rule rotating ring Most of the Seiko slide rule does not have slide rule rotating ring but this one has. Please examine the photos and you'll notice it. Seiko's 6138 was the sophomore release to the 6139 and added a few features lacking on the former. In addition to a rolling 12 hour register for the chronograph, it could now be manually wound at the crown, and the day and date could be quick set independently. With enough design variations to suit nearly every taste, adding one to any collection is an easy decision. Adding this one however, is much easier said than done. The 7000 is easily one of the rarest and most sought after 6138s available. ● Movement Condition - Movement is very clean and works nicely. Chronograph starts, stops and resets perfectly. Movement serviced and oiled by a professional recently. ● Dial Condition - Dial is very clean, no visible fault. Spanish and English dual calendar. ● Case Condition - Case is very good condition, been polished by a professional. Both bezels are in great condition. Outer bezel rotates both ways smoothly. Slide rule rotating ring is present and works. Steel strap will fit up to 23 cm wrist. ● Dimentions - 44mm without crown, very large size for a watch of that era ● Glass - New replacement glass is perfect condition ● Timekeeping - Keeps time ● Production Year - Circa 1970's ● Service History - Serviced by an expert. Will not need service for long period of time ● Shipping - Will be packed with care and shipped insuranced with fast carriers (UPS/Fedex) Usually arrives in 2-3 days. Combined shipping for multiple items. ● Please read description carefully. I tried to explain every detail ● Dear buyer, dial , hands, bezels, buttons and strap are aftermarket. 'This watch contains aftermarket customisations without permission from the brand. This could impact the watch value and might void manufacturer warranties. The brand might refuse servicing and repairs to watches modified by unlicensed watchmakers. Accuracy of + or - 5 minutes per day is acceptable for vintage watches. Water resistant not tested.