A catheter inner assembly includes a receptacle with an interior contour configured to mate with a head and neck contour of a holding member of an implantable medical device； and a laterally facing opening of the receptacle has a profile matching a longitudinal profile of the head and neck. Thus, the opening allows only a properly oriented passage of the holding member therethrough, the passage being without deformation of holding member or opening. The inner assembly further includes a lock and release member useful for securing the device to the catheter, when the holding member has been passed through the receptacle's opening to mate with the receptacle's interior contour. The lock and release member may include an arcuate sidewall moveable between a first location, at which the sidewall overlays the laterally facing opening, and a second location, at which the sidewall is located proximal to the opening.
