From 1967 until 1977, I was the technician on two PDP-1 computers operated inside the National Security Agency. These were serial numbers #35 and #53. Both were in production when I left the Agency in 1977. The article first states there were 52 PDP-1 units built and then later states there were 55 units built. Serial #53 was given that number by Digital Equipment Corp (DEC) when MIT turned in one of their computers and NSA was interested in purchasing it. Since there was extensive modifications to the unit from MIT, it was given a new serial number, 53.
Thank you,
PDP-1guy