[QUESTION]Could anyone identify this seiko?

I got a vintage seiko for pretty cheap and the movement is not working properly. So i sent it to a watchmaker and he said that he cant fix the movement because the case back DOES NOT belong to the case. Which means he needs the matching case back to get the movement fixed. So I'm looking for the name or ref number if possible. If you know which case back is compatible with this seiko,please let me know. Feel free to suggest🙌