Viewmaster Slides for Aircraft Recognition – WWII

This is an interesting item on Trademe.co.nz at the moment – Viewmaster Slides that were apparently ysed by aircrew in WWII for training in aircraft recognition. I had never heard of this before and I wondered if these are well known or pretty rare?

http://www.trademe.co.nz/Collectables/Militaria/WWII-earlier/auction-19429846.htm

I loved Viewmaster as a kid – ah, those balmy days before the Internet…