Well, not really an option to know if it's not disassembled, but it's more likely on the sensor. I had an oil or grease stain on my fuji once, looked similar. Maybe something has dropped from inside the lensbarrel to the sensor or just a big piece of dirt.
Yesterday Oppenheim my grd IV, bought 1 Year before from Japan. Near mint…..
But only from
Outside. From the First day i had doubt in the sharpeness. The pics are soft, shining and Look like vignetting.
Now was terrible with Spots and i Open the Cam. I Look at the sensor, terrible.
You Cam See the sensor at the pic and you can see the reason of the Problem.
The Cam is super But i think you will never find a cam (grd III/IV) with a fine clean sensor because of the Age.
I would be very happy to find a modell in really good conditions. Mint outside (Body) But More important the sensor.
Mike