If you're unfamiliar with the definition, for an oriented smooth manifold $M^n$ we define the inertia group $I(M)$ to be the set of (h-cobordism classes of) homotopy spheres $\Sigma^n$ such that $M\#\Sigma$ is orientation-preserving diffeomorphic to $M$.
I'm trying to compile results into an expository Master's thesis on the subject, and it seems silly to not know the origin. Digging through old papers about the Inertia Group, I'm having a hard time finding the start of the trail. Many early papers refer to Tamura's "Sur les sommes connexes de certaines variétés différentiables" so I expect it to be close to the beginning, but I have been unable to find a copy of this paper.
I am aware of a few members here who are familiar with this theory, and maybe were even around when it started. Does anyone happen to know in which paper/book the Inertia Group originated?