My guess is that only the solid piston head is held with the crimps.
The rod is most likely held in the head by the splines and a cross pin.
Thats the way the Diana 52 barrel is held in (splines and cross pin).
Those crimps are nowhere near deep enough to extend to the piston rod.
That groove in the piston rod can ONLY be there to receive a cross pin.
Either the pin has broken or not been installed at factory allowing the rod to come loose.
If you fancy "crowd funding" a new piston for me Barry, I'll cut one apart Saturday morning
In fact I'd be very surprised if Tony Leach doesn't know the answer to this as he's done the 22mm piston and skirtless kit on enough of them.