https://www.google.com/search?q="voltage+slope+detector"+-patent+-fm&tbm=isch
https://www.google.com/search?q="voltage+slope+detector"+-patent+-fm&tbm=isch
← now the 2-nd such schematic on Earth

 

https://www.google.com/search?q="voltage+slope+detector"+-patent+-fm&tbm=isch
View attachment 114019 ← now the 2-nd such schematic on Earth

At least it may be the second one that is NOT listed in a patent and NOT FM