Interesting, don't think any of our lamps have that.

It's not an usual feature indeed. Interesting that this one doesn't have the mechanical switch that was once common to Westinghouse's safety mercury lamps. I have a 1981 variant of the present lamp and it has that mechanical switch. Maybe Westinghouse changed the lamp design later on to improve the reliability of its safety feature. It was known that lamps with a mechanical switch could still keep on operating and output UVs with a partially damaged bulb, something a series filament prevents.

@Max - I think Westinghouse made both types for a period of time. Philips discontinued the spring cut-out soon after their buyout.

Two technologies for the same lamp denomination... or the art of complicating things unnecessarily. Philips was right to stick to one method only.