Thanks Gerald, if the lens board was loose the artefact #1 on each side would be diffuse, in fact they are very sharp and well defined. It is possible a light leak is contributing to artefact #2.
erikg, I tend to agree with your analysis about the piping with intense light. That could explain artefact #2.
Graham, no I'm using 120 film. Interestingly the pressure plate doesn't actually press tightly on the film edges, it presses on two parallel rails either side and slightly higher than the film rails - see the diagram below I drew.

I'm now pretty convinced in the explanation(for artefact #1) offered by ic-racer whereby the light reflects off the film surface at the edges and bounces off a recess back onto the film outside the frame area - the picture shows it better.

IMAG0403_cross section of Mamiya C330.jpg