The bottom of the manufactured unit had three "vee" slots that mated with three balls on the supporting plate. We designed it that way so that a badly distorted supporting plate could not communicate a twist to the print head, but it also made the whole print head easy to replace in a printer. For some reason, the wires under the unit were retained by a securely attached steel plate, rather than a flimsy aluminum one that would have done the job very well, and introduced another thermal prcision hazard. 020