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yes it is discontinued. It was from a time of the first hub dynamo and halogen lamp that had no switch as the bottle dynamo was the switch originally! But widely available and a rather neat solution.
here for GBP10 shipped (or GBP8 without the connector that you don't need)
https://www.ebay.co.uk/itm/175742699395?hash=item28eb14c783:g:4R0AAOSwLmRj73eh
disconnecting is not really an option, connector won't hold after a week!
Can't find it available anywhere, but that is generally a good idea.
I just got another idea, as rear light is connected to the front one, can I just disconnect the cable between them when I don't need it?