Mart's background in construction here is evident. The transparencies were used so one could be laid over another to determine the the location of materials in relationship to another. For example an E plan would be electrical and an A plan would be architectural. With one on the other the electrician could tell what he had to work around or through. They would, as Mart said, be produced in multiples and sent out for bid to all the trades. Their could be 10 electrical contractors or more bidding on the job. Add the same amount for steel, plumbing, hvac, paint, etc and you are talking about many sets. The material, because of its transparency, is used to copy one or more pages onto paper also.