Wheelgun28 several things occur to me viewing your gun,
First the frame is not mis stamped because it is a 17 frame.

Second its definately an 18 barrel,
Its hard to imagine S&W QC would not to catch a major mistake like that but a letter is the only way to know for sure how it left the factory.

I am guessing the PC Diamond Magnas do not number to the gun?
PC's were usually used on non target frames