The one on the left.
This kind of solenoid was for a limited time available on ebay.
I removed the metal piece around it, because the foot piece was to big for fitting in the case. I think all original ibm solenoids will fit without chopping in the case (right side). Of course now the hammer will have no "cage", so I placed the solenoid directly infront of the right side of the keyboard case. The solenoid will now hammer direct at case (damped with a piece of foam tape).
Here a drawing
The case has a really tight fit, the solenoid is taped down to the top and down with double-sided tape. The driver board is in a plastic bag stuffed inside the case.
More pictures from the keyboard in general. The keycaps are from an typewriter and a model M, also some relegendables.