I have tried to photograph the two Caps so you can see the tiny black plastic 'lugs' that are all that holds the Cap on the Reservoir.
No thread - just a couple of tiny bits of plastic.
Fitting the new one it was clear to me that it was actually hard NOT TO break the tiny bits of plastic as there is no sensation of the Cap "screwing on" to the Reservoir in the way that there has been with every other car I have owned. You have to match the internal 'lugs' on the Cap with the internal spaces in the Reservoir, gently push the Cap down onto the Reservoir, then gently turn the Cap. Any force and you'd break it again. Great design.
New one is on the right. Tiny black plastic lugs are at 5.O'clock and 9. O'clock.
