This was the next generation of the Theratron, designed mainly to reduce cost. The expense of grinding, polishing and painting of the stainless castings (of the older Theratron 80 model) had become prohibitive. Thus the cast steel structure was replaced by a skeletal steel weldment covered with an outer shell of vacuum formed ABS plastic sections. Not only was this much less expensive but servicing was  more convenient, since all covers were removable; and it was no longer necessary to fish through small holes in the castings to access the internal components. -Lawrie McIntosh