Exam 312-49 topic 1 question 224 discussion

Heather, a computer forensics investigator, is assisting a group of investigators working on a large computer fraud case involving over 20 people. These 20 people, working in different offices, allegedly siphoned off money from many different client accounts. Heather responsibility is to find out how the accused people communicated between each other. She has searched their email and their computers and has not found any useful evidence. Heather then finds some possibly useful evidence under the desk of one of the accused.
In an envelope she finds a piece of plastic with numerous holes cut out of it. Heather then finds the same exact piece of plastic with holes at many of the other accused peoples desks. Heather believes that the 20 people involved in the case were using a cipher to send secret messages in between each other. What type of cipher was used by the accused in this case?