I've not dealt with a '49 but if it's the same as my '38 factory knobs are cast on. The wire shaft is permanently bonded inside the knob. Replacements are left to the installers discretion as to how they are bonded. If this is the case with the '49 the slots are simply the manufacturer's way of giving the installer options. Having said all that it's possible the '49 is nothing like the '38 and everything I just said doesn't apply. Hopefully for both of us someone with '49 experience will chime in. Good luck with it.

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