It is authentic. Typical watch out of the early 60ties I guess, maybe a bit earlier.
You got a Stowa Parat with an OS 1065 automatic movement;
not all Stowa watches had Stowa marked movements or rotors.
Osco 1065 (Otto Schlund, back into the watch business as Schlund watches)
automatic movement, 25 jewels
diameter: 11.5''' =25.94mm
If you want to know more about "Stowa Parat" have a look here: