So much tooling from defunct companies has shown up via K-Line, RMT, Williams etc, but the Marx E7 tooling remains a mystery as far as I know. While I have quite a few and love their look, They really were a misfit for modern production. Size wise, they are very close to 3/16 scale, not O. The heavy molded in detail would have been very expensive to rework for a size with no home. My guess is no one wanted it.
Steve