Noel Chiappa pieced together some clues about the PDP-3 and the CASINO
computer, which led me to find this passage in the book "RAINBOW and
GUSTO".

As the amount of radar testing at the Ranch increased, the
scientists at SEI decided that the data had become too much to process
by hand. To speed things up, they decided that a computer would be
needed. The difficulty was that no computer within their budget was
available off the shelf. So they decided to build their own.
In October 1960, the Digital Equipment Corporation (DEC) had
produced a specification for the PDP-3, a new system in their line of
programmable data processor minicomputers. Ed Rawson took charge of
the project and with the help of Chuck Corderman and Jay Lawson
designed and built a PDP-3 using standard DEC logic modules. [EG&G
personnel sometimes teased Rawson that the SEI folks must have held
stock in DEC. (Pendleton, Wayne E., e-mail to author, 14 Feb. 2007)]
Because disk drives were not available, a tape loop running through an
Ampex tape drive held intermediate results; eventually, the tape loop
was replaced with a drum memory. The project was run like a homebrew
computer project, with more emphasis on getting the machine and
software to run rather than on making it well documented and easy to
use. The design evolved so rapidly that when one of the engineers
returned after a two-week absence, he didn’t recognize it (Interview
with Daniel Schwarzkopf, Stow, MA, 30 Nov. 2003). The design evolved
away from the original PDP-3 architecture, and it came to be called
CASINO, for computer able to select internal orders.
Eventually the system worked. Radar data were recorded by EG&G at
the Ranch on 1-in.-wide data tapes and shipped to SEI in Waltham,
Massachusetts. The data could be processed correctly, but the computer
could usually only be operated with Rawson looking over the user’s
shoulder. Eventually the PDP-3 was discarded; one computer engineering
textbook stated that in the early 1980s it was running somewhere in
Washington state, but the author of that book could not confirm it
(Bell, Gordon, e-mail to author, 13 Feb. 2007). There is an
unconfirmed report that it was donated to a Boy Scout troop and
eventually given to Dow Chemical for disposal. It was the only example
of a PDP-3 ever built.