PDP-3
The PDP-3 was an early computer from DEC, described as roughly a 36-bit version of the PDP-1. Supposedly, only one was ever built (and not by DEC).
From Gordon Bell's book Computer Engineering - A DEC View of Hardware Systems Design: "DEC also never built a PDP-3, although one was designed on paper as a 36-bit machine. [...] In 1960 a customer (Scientific Engineering Institute, Waltham, Massachusetts) built a PDP-3. It was later dismantled and given to M.I.T.: as of 1974, it was up and running in Oregon." This was almost certainly the machine called CASINO.
According to Gordon Bell's oral history, after the PDP-3 was designed on paper, one was 'sold' to the Air Force Cambridge Research Lab, U.S. Hanscom field. Since DEC at the time didn't have the resources to build a new 36-bit machine (particularly all the software for it), he and Harlan Anderson persuaded the customer to take two PDP-1's instead!
