Another digging around the 'net in a moment of boredom brought this gem to light
http://www.jogis-roehrenbude.de/
There is all sorts of stuff on here including a page with a lot of info on magic eye valves.
It is in German so translation is going to be necessary for those that don't read/speak German.
I should probably add this warning: there is a historical page on German civilian wartime receivers that may contain insignia and logo's that may distress some people. The website author acknowledges this and does offer to remove such things if it upsets anyone.
Tip: To get the Chrome browsers translation bit to work, on the sites menu up the left side, right click on the section of interest and select "open link in new window", you should get the page in a new window with the pop-up at the top offering to translate the page.
If you use a different browser/translation app/service, do the same thing to get the address of the page to paste in to that service.