Hello, I bought today this nice little Zenith: It has some damage in the dial but otherwise is in good condition. The movement has the serial number 3579287 which according to the following table dates the watch from 1945-1946: Also near the balance I noticed that there is the number 810 which I think is the movement number so it's a Zenith 810. The caseback has the number 8531713 stamped on the outside and in the inside it has the words Zenith and acier inoxydable: I searched for information but I didn't find too much. Even Ranfft doesn't have info about the movement! If you could tell me more about this watch I would be grateful.
You've done your homework and gotten about all the information that's gettable about this watch. It is indeed a Zenith Cal 810 from about 1945. According to Roessler, the movement was made from 1944 to 1955.
If I am not mistaken the number in the caseback, is the reference number. So does Roessler mention this reference;