I don't think the S and T indicate the powder used for proof. I read them as standard Belgian inspector's marks.

The shot and powder markings on the barrels forward of the flats indicate a gun initially proofed between 1897-1903. The choke mark, also forward of the flats with one number over another, came along later (1910-24). Perhaps a reproof at that time. Although unusual to find a 3" 20ga back then.