I have heard it said many years ago, that the way to tell the difference between a hand cut stem and a pre-molded stem is that the hand cuts stem has rounded corners in the opening at the end of the stem. I suppose that could be faked by filing out the square corners with a round Jeweler's file, but it's hard to fake the quality of the vulcanite itself. Having had a number of stems replaced in the 1980s, I noticed substantial difference in the quality of replacement stems, especially as compared to the original stem on older pipes such as Comoys and GBDs.
My experience with replacement stems varied from very good to very poor, depending on who the repairman was. I do hope that has changed over the years with more Craftsman getting into the business, as I do have a few pipes that need some repair work.