I don't see anything like this on the grip frame, and due to the lack of factory protocol used for the re-finish, I believe you're correct; re-finished outside the factory:
Photo thx to Masterpiece.
Boring the chambers all the way thru w/o a shoulder is another clue to non-factory work.
Although some believe th cyl does not have the proper strength for 357, documentation indicates the N frame 38 in addition to the 357 retained the cyl heat treating process when it was discontinued on all other calibers. Still, it might be better to err on the safe side and not shoot 357 in it.
NOTE: Heat treatment was eliminated by order Oct. 12, 1945 for cylinders on the .22 & .32 ‘I’ frames, K22, K32 & K38, and the 44 & 45 N frames, from 'S&W 1857 – 1945'. The 38 N frame is not specifically listed. This likely means that 38/44s retained heat treated cyls, initiated on all model HEs by 1921 and continued thereafter until the 1945 change. And the .357 continued to have heat treatment as well as all subsequent magnum cartridge models. May be cost cutting, improved metallurgy, or more probably, both.