Actually S&W used the same K-frames to build both .38 Special and .357 Magnums, with the exception sometimes for adjustable sights, and a slight modification to allow for a wide or narrow barrel rib. However - and this is very important - the cylinders for magnum revolver were made from a different steel, and went through a different heat treating process, then what was used to make .38 cylinders. Therefore rechambering .38's in a very bad idea.
Over time, and for various reasons the K-frame didn't prove to be a good platform for a .357 Magnum that was used extensively with magnum ammunition. This ultimately led to the development of the slightly larger L-frame.
I'm not sure how a magnum and .38 cylinder can be used interchangeably because the .357 is longer then the .38 Special.
Anyway, since you don't know the full background history, or who did the modification (and in particular what those were) I would pass, and buy a magnum revolver (if that's what you want) that started life that way when it left the factory.