I have a Bolex SBM, and when I looked at your picture of the filter, it appeared a little strange to me. I therefore checked my camera and compared it with yours.
If you look closely at your photograph of the filter, there appears to be a gap above the filter. One can see through this gap. On my camera, the gap is closed by a plate and one cannot see what is behind the plate. It occurs to me that if one can see through that gap then it is possible that light is able to get through it too. Behind the gap lies the prism and if light is able to get to the prism it seems to me possible that it is able to get into the top of the prism and thence to the rear face of the prism. Behind the prism is the shutter. Now, if light is contuously getting to the shutter, it is possibe that it is somehow getting to the edges of your film. The pressure pad pressing the film against the gate might be preventing the light affecting most of the picture area, but on the edges of the film, the pressure pad might not be quite achieving that.
I also took a look at pictures of the mount area of an SBM camera which appears in Andrew Alden's book "Bolex Bible" (well worth getting if you do not already have a copy), and the gap to which I refer above appears to be blocked as it is on my camera.
I hope this helpful, and apologies in advance if I am wrong.