17.1 is the bore diameter (16 gauge) expressed in MM. PT means it was proofed with French smokeless powder T, which came into use in 1900. The crossed lightning bolts mean it was proofed in a finished state. It appears to have the 1894 rocker style of safety on the left side of the gun. Certainly appears to be brazed barrels, as opposed to mono-block.
Looks to be an enjoyable bird gun. Use it in good health.
Best,
Ted