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This picture shows the light sensors and circuitry of the dual tape readers. I find both the early 1940s original Colossus and the replica to be remarkable. It is amazing what they were able to create in 1943 with little prior experience of digital electronics. Some could claim that Colossus was not a full fledged computer, but I would counter that all of the early computers lacked portions of a modern computer. It took another 10 to 15 years for a computer as we know it today to be technologically possible. In particular, it took a long time for a practical random access memory technology to become available for program storage. It seems that Colossus had data storage so that the logic could see the current and four previous message characters. That's 25 bits of memory. The documentation I've seen on the Colossus memory isn't fully clear to me, but it seems that each memory bit used two thyratron vacuum tubes. It also seems that thyratron shift registers (memory) were also used to sequence each possible code wheel setting into the decryption logic. Colossus did not have a full-fledged arithmetic processor; only a binary half-adder was needed for decryption. Colossus did not have conditional program sequencing (if-then-else). Colossus did not have a way to store a program, that was the responsibility of the human operator and cryptologist. Colossus was a remarkable accomplishment. It was probably 20 times larger than any previously built electronic equipment. That led to electronic general purpose computers after the war, as well as large electronic telephone switching systems. Colossus showed the feasibility of high speed paper tape as a data storage medium, which led the way in later years to high speed magnetic storage which we still use every day, our hard drives. About ten Colossus computers were built before the end of the Second World War. All of them were eventually destroyed and their existence was kept secret for decades. However, some of the individuals familiar with the technology were after the war involved in computer developments in academia and industry in Britain and to a lesser extent in the US. If you have further interest, I found the book "Colossus, Bletchley Park's Greatest Secret" by Paul Gannon to be very good.