Work done by Schmieden&Laugwitz in the 1950s helped to formalize and retroactively apply rigor to the earlier uses of infinitesimals. Their Omega Calculus approach was also constructive.

cf. Detlef Laugwitz, Curt Schmieden's Approach to Infinitesimals. https://doi.org/10.1007/978-94-015-9757-9_12

However this turned out less useful going forward.