The stereospecificity of the olefin-keteneiminium salt reaction is shown to be dependent on the nitrogen substituents. This is taken as evidence for a stepwise mechanism.
Saimoto, H., Houge, C., Frisque-Hesbain, A.-M., Mockel, A., & Ghosez, L. (1983). Nonstereospecificity in the Cyclo-additions of Keteneiminium Salts To Olefins - Evidence for a Stepwise Mechanism. Tetrahedron Letters, 24(22), 2251-2254. https://doi.org/10.1016/S0040-4039(00)81896-1 (Original work published 1983)