Does anyone know the first times the term plinth ideal of a $ \mathbb{G}_{a} $-representation was used?

Question

Let $ X $ be an affine, normal, $ \mathbb{G}_{a} $-variety with action $ \beta $ over a field $ k $ of characteristic zero. The action of $ \mathbb{G}_{a} $ is obtained from $ \delta \in \operatorname{Der}_{k}(\mathcal{O}(X)) $ via \begin{align*} \beta^{\sharp}(f) =\exp(t\delta)(f)\\ = \sum_{j=0}^{\infty} \frac{\delta^{j}(f)t^{j}}{j!}. \end{align*} The plinth ideal is the ideal of $ \mathcal{O}(X)^{\mathbb{G}_{a}} $ generated by all elements $ h $ such that there exists a $ g \in \mathcal{O}(X) $ such that $ \delta(g)=h $. The first paper I could find with this name usage was one by Dufresne and Kraft (Invariants and Separating Morphisms for Algebraic Group Actions). Does anyone have any earlier references with this name usage or concept?

Answer 1

G. Freudenburg, in Algebraic theory of locally nilpotent derivations introduces the term "plinth ideal" in a way that suggests it was not used before, on page 10, with the footnote:

The term plinth commonly refers to the base of a column or statue.

S. Kuroda, in arXiv:1412.1598 writes that "the notion of plinth ideal already appeared in Nakai (1978), although not called by this name."