Navigators Pen?

Hello all
In August 1940 my grandfather was first on the scene with his dogs and a pitchfork when a Dornier 17 (U5-EA) crash landed in a field on the farm where he was gamekeeper at Wickhambrook in Suffolk.

I can remember him telling me that one of the crew gave him a pen that illuminated the writing area when you started to write. I wondered whether anyone has ever heard of such a thing? I've certainly not found evidence of anything like this via google searches etc., On the face of it, the idea would seem to make a lot of sense, but I'd be very interested to hear if its something you've ever come across ?
Many thanks in advance for your kind input.