Citizen’s pioneering first ‘Electric Watch’ was launched in 1966, and used the 25 jewel 0800 electro-mechanical movement, running at 18,000 beats per hour (see the Chrono Master model here: https://sweep-hand.org/2011/07/11/this-weeks-featured-watch-10-the-electric-watch/)
As the technology developed Citizen introduced the next version in 1970, now called the Cosmotron X8 and with the 12 jewel 0840 movement (designated 4840 with date complicaton). As with the jewel count the price was coming down too, from over JPY30,000 to under JPY20,000. Here’s my example, from May 1972:
The case back shows the usual information, but marked ‘electronic’ of course, with serial number confirming the date of production:
I don’t think this is the original bracelet since it’s a folded type, and does not have curved ends (the 1971 catalog shows this model on a leather strap), but at least the clasp is signed with the Cosmotron logo:
The movement is signed and the jewel count is shown. The battery for these is 1.55 volts, as with the earlier 0800 movements. The later 7800 movements use 1.3 volts power. The 0840 beats at 21,600 beats per hour, so again we see development in that area, which was upped to 36,00 beats per hour in the 7800 series, and peaked at 43,200 beats in the 5800:
Sweephand's Vintage Citizen Watch Blog 
Hello!
Do you check the cosmotron movement daily variation?
How many seconds can be the best result after service and adjusment of the cal 4840?
I’m watching my Cosmotron in this case, I will wait the conclusion.
regards
István
Hi Istvan, thanks for visiting my blog – I’m sorry for slow reply. I’m not a watch repairer so I don’t have the tools to check daily variation. As far as I know the Cosmotrons achieve the same results as high beat mechanical watches, so a few seconds + or – per day should be achievable for a movement in good condition. The later models, with the 78xx movements should be able to run close to chronometer accuracy.
Stephen
Hello!
After service (by my watchmaker) accuracy was -5 sec. per day.
Some days later at home I tried to set the fine tuning screw on the balance and the result is -1 sec per day!
I think it is a good accuracy for a 43 years old electro-mechanical movement.
regards
István
That’s great accuracy Istvan – I’m not surprised though 😉 Providing parts are’t really worn, Cosmotrons are usually very good. Their design means that wear on the moving parts is not great over the years, less than a conventional mechanical watch I believe.
Stephen