To me , this looks like a direct copy of the hw piston set up , but they have forgotten the pin, if there was a pin you would see the top of it ( it has to be inserted from somewhere) I think the splines are only there to stop the rod turning while its being assembled .
If Diana have been economical and got rid of the pin and associated operations , and are relying on the crimp, no way is the crimp ever going to deform into the rod groove imo the pressure invoved to do this would deform the piston head .
Easy fix is to drill a 4mm hole throug it and pin it, hw use a hardened pin , walther in the lgu use a silver steel one, both similar set ups with the same diameter rod so take your pick.