Impossible Dovetails

I was shown this joint at Yandles show. At first glance it looks like a lapped dovetail until you look more closely, it's angled at the top as well.

It was made many years ago out of boxwood and rosewood, both very hard and yet it was still a perfect fit. The pictures below should explain how it was achieved.
Apparently it was a joint used for holding together the top of large bookcases.

Another fine craftsman showed me some dovetail guides he had made and they really were things of beauty!

The two pieces were dovetailed square and then the angles on the sides cut afterwards. He was thinking of adding magnets and using them as a magnetic dovetail guide. Not that he needs one as these were cut totally by hand and were perfect.