I have no idea about Citizen serial numbers. But the back doesn`t come off the square one! Inside the top edge of the case back which houses the movement, very close to the winding crown, you will find a `trigger`that is evident when you pull the winder out to a hand setting position. Push that trigger down, pull out the winder, invert he case, and the movement should fall out. I would guess the age of the watch to be circa late `60s or early `70s. It`s none of my business, but what are your intentions if you get it out of the case.

The Citizen serial numbers do not pinpoint the decade, they decode to give the last digit of the year and the month. Select as many other options as you can to get the best chance to narrow down the exact date. If the guarantee is photographed check that any model or serial numbers match those featured on the caseback of the watch. Also check carefully for any errors in logos, spelling and dates.