Ian;

My point was that the SO3-- reacts to form H2SO3 and SO2 reacts to form H2SO3 (Sulfurous Acid) in acidic solution. If pure SO3 itself is present, then you get H2SO4. The main difference is the final pH and the presence of Sulfuric Acid from any nonionic Sulfur Trioxide which would be minimal in any case.

I think it is mainly the amount of Sulfite present as a preservative and silver halide solvent that is notable here. That is my own opinion though.

PE