The '66 250S that I did had the early type ignition switch - the one with the 5 screw-on wire terminals.
The plastic/phenolic switch back was crimped into a short, round alloy casting. The casting had 3 ears that fastened with 3 slotted screws to the end of the steering-lock barrel.
Usually, the way these switches fail is that the crimps on the alloy part of the switch lose their grip on the plastic part. Often, the plastic part will actually fall off! You still need to unscrew the metal half of the switch from the lock-barrel. I beleive the key and tumbler is removeable from the outside, without needing to remove the whole steering-lock assembly.

Many years ago, not wishing to fork over the $60 or so to buy a new switch for a Benz that I was selling anyway, I re-crimped the old one and it worked just fine.

Happy Motoring, Mark

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DrDKW