ETA/Unitas caliber 6503 is the rarely seen version of the manual wind ETA 6497 with a date complication.
The 6503 was produced in the 1960s and is currently discontinued. It is hard to find watches powered by this movement, if you have a timepiece with the 6503, please share it in the comments below.
There is a German watch company called Guinand which still produces a manual wind pocket watch featuring a calendar window at 3:00 (Model G-1996S). Guinand’s caliber HS81 is based on the ETA/Unitas 6503, but with modifications created by Helmut Sinn, including a central sweeping seconds hand (instead of the standard 6497 small seconds at 9:00), as well as a hacking mechanism. It is likely that they had access to new old stock 6503 movements for this model.