As Lagopus says, the black powder only proof marks take it to before 1887 but the W&S Bullet logo is post 1897. It could have been bought back in by W&S after the amalgamation and then resold. I don't know for a fact that they stamped up pre-owned guns that they had through their workshops but it is possible.
One other solution is that it was very old Webley stock that was proofed back pre-1887 but never sold. When W&S formed they certainly stamped up old Webley stock before sale. I know because I have handled a couple.
We will probably never know.
I don't have a problem with polished barrel flats with a gun of this vintage. Birmingham marks of this era were more likely to be on the barrels, leaving the flats plain.
