That's certainly interesting with those helmets. I had only seen the visorless as well.
I think Sir Ian is correct. A klappvisor's visor is easily removed, and would probably need to happen to fit the helmet over the head. I can't see it possibly fitting anywhere near correctly with the visor on too - not to mention it also doubles the vision restrictions, looking through visor slots inside of visor slots.
Careful with Wikipedia as a source. It's not always reliable, and is sometimes just regurgitated from another site, which may be incorrect. If they cited that, check the original source if you can.