During annual inspection of my HF antenna system, I removed the lid from the common mode choke.
The choke is housed in a (German) Hensel PS electrical terminal box. Hairline cracks were observed radiating out from one of the terminal screws, and adjacent to both screws in the lid. The lid is obviously not strong enough to be secured by two screws in the way done.
A similar choke that has not been in the weather does not show such severe cracking (though one tiny crack near a lid screw), so it is probably a fundamental design failure exacerbated by exterior use.
Whilst it was apart, the lid was drilled and tapped M12x1.5 to take a Gore hydrophobic vent plug which helps to reduce the stress on seals (and cracks) to some extent, so given that the choke is located under the roof soffit, it was returned to service to be built into a new enclosure at a later time.