Advice needed: Dust problem (?) with SX-70

Hi All,

I've recently started to get a nasty defect on my Polaroid SX-70 shots. It's always in the same place and looks very much like a dust problem. You can see an example below. I've cleaned the lens on the outside and I've also removed the plastic shutter cover and blown some compressed air around.. but I can't see anything on the lens anyway.
Could it be on the mirror? How would I clean the mirror? I would rather not disassemble the SX-70 as that looks a little complex.
Any advice would be appreciated.

The mirror is quite off the image plane of focus.

Could there be something hanging inside the camera as a fibre with someting attached to it?

Anyway, flipping off the ejection assembly gives you a good sight into the body.

Thanks, I'll take a look from underneath.