Dunno really...any guesses?
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Hmmm... me thinks these windows are in front of your shutter? The shutter is just in front of the final (rear) element. Looking inside the bellows towards the lens? From what I saw, the leak looks central for sure, but at the bottom (as leak was top and semi-centered on print). Bellows or pack door seal are perhaps more likely? But, as you note, these are just guesses!
Plus, it also looks like a bit is missing from your front element surround, this too could be a problem? See below for what I expect a 250 to look like:
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