Systems for effecting transfer of reaction receptacles between modules of an instrument for performing multi-step analytical procedures and for adjusting for variations in the positions of such modules includes a receptacle carrier supported on a track on which the carrier translates between different modules located adjacent to the track. The carrier includes a receptacle distribution head supported on a receptacle carrier carriage. The distribution head includes a receptacle moving mechanism adapted to move receptacles into and out of the distribution head and into and out of a module. The carriage is engaged with the track and adapted for translation along the track. Drive systems effect powered translation of the carriage, powered elevation and rotation of the distribution head, and powered movement of receptacles into and out of the distribution head. A transfer position locating system automatically determines a receptacle transfer position of the receptacle carrier with respect to each module.