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Mike and I know all about that.
Put a very thin steel sleeve over the handlebar clamp area, or take a punch and pound some divets in it. It will induce some elevated nubs that the clamp will grab into and keep the handlebars tight.
All the bars I have, have the knurling. Sometimes it isnt enough, or they get worn down. If you rechrome your bars they don't have deep enough knurls. The pointed tip chisel displaces some of the metal and causes bumps and the clamp has plenty of grip then. Cures the problem. You can make a shim that has these bumps and sleeve the knurled area if it's super worn out. Either method is relatively easy to do and fixes the problem. Next time, I'll take before, during, and after pics.
That may be...and yes, most bars have knurling. The bars used with these stems are a bit different, as @Igrinnings posted. They are earlier bars, fazed out in about '38 or so as were the stems. I have several '36 Colsons with this exact set up and a '37 Monark. I'm very careful not to overtighten them or put much weight on the bars while riding. I have yet to break one.
Being careful is key with that cast stem. They are delicate and not of the best design. I have original handlebars from the late 40's that have the sleeved knurling originally. As with any stem, if the bars start moving, it gouges into the knurling, and if not remedied soon, you'll have to fix the issue anyway you can.
