Thank you for that reference.
I was surprised to read Malthus wrote about this in 1798, and it really does sound like it but I could not find the ‘K level’ there, however I did find “Slope is the marginal product of capital, MPk. This is because adding one unit to k, the quantity of capital per worker increases y, output per worker, by the marginal product of capital because f(k)=F(k,1)
As the slope is MPk and because MPk is diminishing with K, the production function is concave (its slope decreases as k increases)” which sounds something like it. Yes, this could be it and why my professor was so surprised.
LOL you lost me after 1789, but I did learn about this theory in college without going deeply into the math part.